| Abstract: | JIS SKD11冷作鋼非常適合用於模具或機械工具之製造業上，然而，由於該鋼材之模具或機械工具之缺點為壽命短，因此使它們的使用量大大降低。因此，本論文之研究目的在常壓電漿噴射束(Atmospheric Pressure Plasma Jet, APPJ)之系統，通入300 sccm-H2/N2之混合氣體針對JIS SKD11冷作鋼進行滲氮，以探討電漿所提供之溫度與製程時間，將控制表面硬度與滲氮厚度之結果。
本實驗採用常壓電漿滲氮(Atmospheric Pressure Plasma Nitriding, APPN)系統，在SKD11冷作鋼於483.5℃ (400 W)、544.5℃ (500 W)、628.0℃ (550 W)和705.6℃ (600 W)進行。滲氮後之試樣，透過分析量測可得滲氮層的顯微組織結構、化合物層之厚度、擴散層之厚度、表面硬度與表面形貌之變化，並且透過利用X射線繞射(X-ray diffraction, XRD)與高解析度場發射掃描式電子顯微鏡(Field Emission Scanning Electron Microscope, FE-SEM)分析之外掛之能量色散X射線譜(Energy Dispersive Spectroscopic, EDS)進行Mapping分析，以此瞭解表面微觀結構的變化。XRD分析滲氮過後之試樣，發現表面存在氮化相分別為ε (Fe2-3N)、γ’ (Fe4N)與CrN，此外，SKD11中的氮濃度隨著製程溫度的升高而增加，而表面硬度隨著製程溫度的升高而上升，於製程溫度544.5℃以內時；當製程溫度超過544.5℃時，滲氮層靠近表面處會形成具有孔隙的滲氮層，因此導致表面硬度下降。在本實驗中，具反應性的電漿物種能夠透過光放射光譜儀(Optical Emission Spectroscopy, OES)進行物種之量測，以此分析H2/N2的工作氣體產生通入常壓電漿中形成噴射束，透過電漿診斷確定活性電漿之物質，因此得知在300 sccm-H2/N2之混合氣體中，NH基對滲氮之過程扮演活性物質的角色。
APPN製程結果證實SKD11冷作鋼中的氮含量隨著滲氮處理時間增加而增加，而形成更厚的化合層與擴散層，此外，SKD11冷作鋼的表面硬度會隨著升高製程溫度及縮短製程時間或是降低製程溫度及增長製程時間而增加。另外，將製程溫度、製程時間、表面硬度與滲氮層作為參數因子之函數進行更高階的非線性建模，並以此參數研究，探討APPN滲氮參數對底材表面硬度與滲氮層的影響。經過高階非線性模型進行估計新的製程參數，而後透過實驗驗證，可得到估計值與實驗結果之誤差範圍為-15%至6%之內。
經過磨耗測試後，APPN滲氮後的試樣表現出優異的磨耗性能。尤其對於電漿功率500 W與處理時間為10 分鐘下，其磨耗率約為原始底材的十分之三，以此證實APPN會提高SKD11冷作鋼的耐磨耗性，並降低摩擦係數。此外，此項研究還分析滲氮後之試片的磨耗機制，並透過FE-SEM觀察磨耗痕跡在不同功率和製程時間下之磨耗的演化，並顯示磨耗表面呈現磨損性、黏著性、分層性、裂痕和氧化磨耗之模式的結果。
JIS SKD11 cold working steels are widely used in dies and mechanical tools manufacturing. Unfortunately, their use is greatly reduced due to their short lifetime. Thus, this research is conducted to investigate the nitriding process on surface hardness and nitriding thickness of JIS SKD11 cold working steels using 300 sccm-H2/N2 mixture gas in an atmospheric pressure plasma jet (APPJ) system, which is highly dependent on the process temperature and process time.
SKD11 cold working steels were nitrided at 483.5℃ (400 W), 544.5℃ (500 W), 628.0℃ (550 W), and 705.6℃ (600 W) by atmospheric pressure plasma. After nitriding, the microstructure, compound thickness, diffusion thickness, and surface hardness of the nitriding layer, as well as the surface morphology were accordingly determined. Using X-ray diffraction (XRD) and field emission scanning electron microscope (FE-SEM) coupled with energy dispersive spectroscopic (EDS) for mapping analysis was used to comprehend the surface modification of the nitrided microstructure. XRD analysis showed the presence of ε (Fe2-3N), γ’ (Fe4N) and CrN phases of iron was evidently observed. Besides, the nitrogen content in SKD11 increased as the process temperature increased, whereas the surface hardness increased with the increasing process temperature within 544.5℃. The results indicate that the nitrided layer of the surface with the pores phenomenon, can be formed when a high nitriding process temperature was used. The surface hardness decreased with the increasing process temperature when the temperature was over 544.5℃. In this research, reactive plasma species were able to recognize with optical emission spectroscopy (OES) studies. An atmospheric pressure plasma plume produced using H2/N2 gas was explored for the active plasma species. In the 300 sccm-H2/N2 gas mixture, NH radicals play the role of active substance on the nitriding process.
The process of APPN results showed that nitrogen content in the SKD11 cold-working steel extended with increasing nitriding process time and process temperature, and thereby, the formation of thicker compound zones and diffusion zones. Furthermore, the surface hardness of the SKD11 cold-working steel increased by increasing the plasma process temperature and decreasing the process time, or decreasing the plasma process temperature and increasing the process time. Also, higher-order nonlinear modeling of the process temperature, process time, surface hardness, and nitriding layer as a function of parameter factors has been performed. It was possible to carry out parametric research to investigate in detail the effect of each atmospheric pressure plasma nitriding parameter (process temperature and process time) on the surface hardness and nitriding layer of the substrate. The new parameters are estimated by the higher-order nonlinear modeling, and the experimental verification was carried out. The error range between the estimated value and the experimental result was -15% to 6%.
After the wear test, the specimen nitrided with APPN showed excellent wear behavior. The wear rate is found approximately three-tenth of the pristine substrate for 500 W of plasma power and 10 mins of treatment time. APPN improved the wear resistance and decreased the coefficient of friction of SKD11 steel. Furthermore, this research included an analysis of the wear mechanisms of the nitrided sample, founded on FE-SEM observations of the wear track at various power and process time of the development of wear. The FE-SEM investigation of worn surfaces showed marks for abrasion, adhesion, delaminsation, crack, and oxidative modes of wear. |
