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Microstructure and Mechanical Properties of Nitride Coatings on a Type 304 Stainless Steel
|關鍵字:||Nitride;氮化物;TiN;CrN;TiAlN;Mechanical properties;Microstructure analysis;AISI 304 stainless steel;氮化鈦;氮化鉻;氮化鋁鈦;機械性質;微結構分析;AISI 304不銹鋼||出版社:||材料工程學研究所||摘要:||
由X光繞射結果發現由上述兩種鍍著技術所被覆之氮化物鍍膜皆具有(111)優選方向。由穿透式電鏡橫截面(Cross-section)試片的觀察可以發現氮化物鍍膜具有斜柱狀晶結構(Tapered columnar structure)，在鍍膜與不銹鋼基材界面附近呈現微細晶粒，而隨著鍍膜厚度的增加，在鍍膜表面有最大的晶粒尺寸。此外，由中空陰極放電離子鍍著法所鍍著之鈦介層具有柱狀結構，而由陰極電弧離子鍍著法所鍍著之鈦介層則為等軸晶結構。在鈦介層與不銹鋼基材間存在著鐵鈦介金屬化合物如FeTi及Fe2Ti等相。在中空陰極放電離子鍍著法所鍍著之氮化鈦-鈦雙層鍍膜中，發現氮化鈦的晶粒延續柱狀的鈦晶粒成長。此現象亦存在於陰極電弧離子鍍著法所鍍著之氮化鋁鈦-氮化鈦雙層鍍膜中。另外在氮化鋁鈦鍍層內發現氮化鋁鈦具有週期性7.7 nm的超晶格結構。
對於陰極電弧離子鍍著系統，針對基材偏壓及基材與靶材距離探討其鍍著參數對氮化鈦鍍膜微結構及硬度的影響。由實驗結果得知在不同偏壓下(-100V至-300V)鍍膜厚度及硬度值隨著距靶材距離減少而增加，其硬度值介於2200 40與1880 10之間。
微粒的產生為陰極電弧離子鍍著製程特有的現象。本研究針對氮化鈦、氮化鉻以及氮化鋁鈦鍍膜中的微粒，以上述之分析儀器加以研究探討。研究結果顯示氮化物鍍膜中的微粒外觀呈現花蕾的形狀，金屬核心位於內部下方的位置，外面包圍著一層氮化物鍍膜。由電子能量耗失能譜分析發現，在氮化鈦微粒中，金屬鈦含氧量高達22 at.%；而外層的氮化鈦則包含有TiN0.26、 -TiN0.3、 Ti2N及TiN等相的存在。在氮化鉻微粒的分析中，金屬鉻含氧量為4.6 at.%；而周圍的氮化鉻層，從內部到微粒表面，氮及氧的含量分別從1.8增加至38 at.%及5.2增加至9.5 at.%。從選區繞射的結果發現外層的氮化物只有CrN相存在。基於上述實驗的結果，提出一個微粒形成的機制，與文獻上認為微粒為純金屬組成有不同的解釋。
Nitride coatings of TiN, CrN and TiAlN deposited by hollow cathode discharge ion plating (HCD-IP) and cathodic arc ion plating (CAIP) techniques with or without interlayers ( Ti or Cr ) on a type 304 stainless steel are studied. Microstructure and chemistry of the ion-plated nitride coatings were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM), Auger electron microscopy (AES) and transmission electron microscopy (TEM) with electron energy loss spectroscopy (EELS). Mechanical properties of the TiN coatings deposited by HCD-IP and CAIP techniques were evaluated by tensile and hardness tests.
X-ray diffraction results show that the nitride coatings deposited by either HCD-IP or CAIP techniques have strong (111) preferred orientation. Cross-section TEM observation reveals that the nitride coatings have a tapered columnar structure growing from fine equiaxial grains near the coating-steel interface to large columnar grains near the coating surface. The Ti interlayer deposited by HCD-IP technique has columnar structure, but equiaxial grains exist in the Ti interlayer deposited by CAIP technique. It is also found that the FeTi and Fe2Ti intermetallic compounds were formed between the Ti-steel interface. Good epitaxial relations are observed in the TiN-Ti bilayer deposited by HCD-IP technique and in the TiAlN-TiN bilayer deposited by CAIP technique. The TiAlN coating has a supperlattice structure with periodicity about 7.7 nm.
Mechanical properties of HCD ion-plated TiN coatings with non-uniform film thickness on steel wires were evaluated by tensile test. The failure mechanism of the TiN films upon tensile loading is found to be dependent on the coating thickness. Opening mode prevailed in the thick coatings where periodic cracks perpendicular to the tensile axis developed; on the other hand, shearing mode took place in the thin coatings where cracks inclined to the tensile axis by 45. On the basis of the experimental observation and the theoretical calculation, the shear strength of the TiN, the ultimate shear strength and the bonding strength of the TiN-steel interfaces were estimated to be 15.36, 9.87 and 1.06 GPa, respectively.
In the CAIP system, two important parameters (substrate bias and specimen-to-target distance) were studied to understand their effects on the microstructure and hardness of the TiN coatings. The results show that the thickness of the TiN coatings increases with decreasing specimen-to-target distance. The hardness ranging from 2200 40 to 1880 10, decreases with specimen-to-target distance for different substrate biases ranging from -100 to -300 V.
Formation of macroparticles in the coatings is a characteristic of the CAIP process. Macroparticles of TiN, CrN and TiAlN in the nitride coatings were studied by SEM, plan-view and cross-section TEM with EELS. The results show that most of the macroparticles in arc ion-plated nitride coatings have the shape of a bud, in which the metal locates at the bottom center, and the outerlayer consist of nitride coating. By EELS analysis of the TiN macroparticle, it is found that the core region consists of metallic Ti with high content of oxygen, up to 22 at.%; while the outlayer is composed of a wide variety of titanium nitrides including TiN0.26, -TiN0.3, Ti2N and TiN. In the analysis of the CrN macroparticle, the content of oxygen is 4.6 at.% in the core region of Cr. In the surrounded CrN layer, the atomic concentrations of the nitrogen and oxygen increase from 1.8 to 38 at.% and 5.2 to 9.5 at.% from mid-layer to surface of the macroparticle, respectively. It is only the CrN phase identified by selected area diffraction (SAD) pattern in the outer layer of macroparticle. A formation mechanism for the macroparticles is proposed based on the analytical results, which is different from the conventional argument that the macroparticles consist of only metal phase.
|Appears in Collections:||材料科學與工程學系|
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