Please use this identifier to cite or link to this item:
Microstructure and Thermal Stability of Physical Vapor Deposited Chromium-based Coatings
|關鍵字:||chromium nitride;氮化鉻;chromium carbide;hard coating;microstructure;oxidation mechanism;碳化鉻;硬質薄膜;微結構;氧化機制||出版社:||材料工程學研究所||摘要:||
從XRD與平面TEM繞射圖分析得知利用陰極電弧沉積技術鍍著之鍍膜僅含有CrN相，而封閉式磁控濺射系統沉積之鍍膜則混合CrN 與 beta-Cr2N兩相。兩種不同設備所沉積之氮化鉻層均於500℃以上開始氧化而在表面生成氧化鉻（Cr2O3）相，在陰極電弧鍍著之試片在此溫度以上亦出現一新的b-Cr2N相，Cr2O3及beta-Cr2N兩者的含量隨著氧化溫度昇高也隨之增加。同時，不同設備所沉積之CrN在500℃時開始相分離而轉換成beta-Cr2N相；在800℃、60分鐘氧化後，利用磁控濺鍍沉積之氮化鉻鍍膜已完全轉換成b-Cr2N相，而利用陰極電弧沉積技術鍍著之鍍膜仍呈現兩相混合。
由SEM的二次電子影像中得知鍍膜表面在600℃氧化後，鍍膜表面出現顆粒狀的形貌，粗糙度也隨氧化溫度升高而增加。由氮化鉻鍍膜橫截面TEM試片影像中得知兩製程所鍍著且未經氧化之試片為具有柱狀晶結構之氮化鉻層及鉻中間層，最底層為304不鏽鋼基材；而鍍著試片經高溫氧化後於鍍膜表面生成一層等軸晶結構之氧化鉻層，鍍膜本身由柱狀晶結構轉為等軸晶結構之CrN與b-Cr2N兩相混合層；同時，中間層鉻層亦由柱狀晶轉為等軸晶結構。隨氧化溫度升高，等軸晶晶粒尺寸亦隨之成長。此外，利用陰極電弧鍍著之氮化鉻層經800℃高溫氧化後，接近表面之晶粒尺寸約為鍍膜底層晶粒的3～4倍。由不同氧化溫度之歐傑電子縱深分析得知元素分佈且利用其結果換算出不同溫度之氧化鉻厚度，亦由此計算出氮化鉻氧化的活化能為1.63 eV (157.3 kJ/mol)。由電子能量耗失儀(EELS)及歐傑電子能譜儀(AES)分析鍍膜表面，隨氧化溫度升高鍍膜表面的O/N比值及氧化層厚度隨之增加。
接著利用封閉式磁控濺射系統鍍著含鉻多層鍍膜於304不鏽鋼基材上並於空氣中做300℃～700℃氧化實驗60分鐘，由橫截面TEM試片分析剛鍍著、未氧化之試片得知此鍍膜可清楚分成四層結構，由表面至基材依序為厚度為1.25 mm的非晶質碳化鉻層、厚度為660 nm的柱狀晶氮化鉻中間層、厚度為120 nm的鉻中間層及最底層的304不銹鋼基材。而試片經氧化溫度於600℃時由XRD結果得知有兩個新的結晶相Cr2O3及Cr3C2出現，且都隨溫度上升而增加其含量；由SEM二次電子影像觀察得知在此溫度時表面形貌產生明顯改變，800℃氧化時鍍膜已出現局部剝離現象。
由700℃氧化試片之TEM橫截面試片分析得知表面已生成一層厚度約250 nm的氧化層，其晶粒亦隨溫度上升而成長；原非晶質柱狀晶結構的碳化鉻層在此溫度下仍維持其柱狀結構，但柱狀晶結構內之非晶質碳化鉻相已轉為等軸結構之Cr3C2結晶相；而氮化鉻及鉻層兩層中間層經高溫退火後其分析結果則如前述之氮化鉻/鉻/304不銹鋼系統高溫氧化後之現象雷同，由柱狀晶結構轉為等軸晶結構。 平面TEM試片觀察顯示碳化鉻鍍膜的氧化初始於500℃且始於在柱狀晶晶界處，而700℃氧化後的次表層的分析亦顯示出碳化鉻層已轉為結晶性良好之Cr3C2相。 再者，利用歐傑電子縱深分析、鍍膜表面電子能量耗失儀及化學分析電子儀再次確認鉻在碳化層及氧化層的鍵結態分別為Cr3C2及Cr2O3。
Characterization of the Cr-based hard coatings, chromium nitride and carbide, oxidized in air at temperatures ranging from 300 to 800℃ for 60 min were carried out by X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), electron energy-loss spectroscopy (EELS), X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES).
The chromium nitride coatings were prepared by cathodic arc ion plating (CAIP) and closed-field unbalanced magnetron sputtering (CFUBMS) techniques on a type 304 stainless steel with a Cr interlayer. The analytical results of XRD show that the as-deposited coating prepared by the CAIP technique is consisted of a CrN phase, whereas that by the CFUBM technique is a mixture of CrN and b-Cr2N phases. Oxidation of the nitride-coated steel above 500℃ produces a new phase, Cr2O3, in both cases while the b-Cr2N phase is present only in the CrN coating prepared by CAIP. The amount of both phases increases with the oxidation temperature. The phase decomposition of CrN into b-Cr2N occurred at and above 500℃ in the coatings and completely transformed into the b-Cr2N phase for the CFUBM deposited nitride layer oxidized at 800℃ for 60 min On the contrary, it is composed of a mixture of the CrN and b-Cr2N phases in CrN coating deposited by CAIP and oxidized at 800℃ for 60 min.
A noticeable change in the surface morphology of both coatings were observed by SEM in the specimens oxidized at temperature above 600℃. Cross-sectional TEM reveals that oxidation of the nitride-coated steel at elevated temperatures produces an oxide layer, Cr2O3, on the coating surface, and the underlayer is a mixture of CrN and b-Cr2N phases in both cases. Unlike the as-deposited specimens, the dual phase layer in the oxidized specimens have an equiaxed grain structure and the average grain size of the layer increases with the oxidation temperature. In addition, pronounced grain growth in the dual phase layer near the coating surface is observed in the CAIP prepared specimen heat-treated at 800℃. Auger depth profiling of the oxidized CrN coatings prepared by CAIP at various temperatures reveals the elemental distributions in the coatings and the thickness of the oxide layer near the free surface, from which the activation energy of oxidation for the nitride coatings is calculated to be 1.63 eV. Elemental analyses of the CrN coating near the free surface by EELS and AES reveal that the O/N ratio of the coating and the thickness of the oxide layer increase with the oxidation temperature.
Subsequently, microstructure and chemistry of the Cr-containing multilayer carbide coatings oxidized in air at temperatures ranging from 300 to 700℃ for 60 min were studied. The Cr-containing multiplayer coatings were prepared by CFUBM technique on a type 304 stainless steel with a Cr (~120 nm thick) and a chromium nitride (~660 nm thick) interlayers. The XRD result shows that oxidation of the Cr-C coated steel above 600℃ produces two crystalline phases, Cr2O3 and Cr3C2, and the amount of both phases increases with the oxidation temperature. From SEM images, a noticeable change in the surface morphology of the coatings is observed in the specimen oxidized at temperature above 600℃ and partially spalled off occurred in the specimen oxidized at 800 ℃.
Cross-sectional TEM shows four distinct regions including the steel substrate, Cr and the nitride interlayer and an amorphous Cr-C coating in the as-deposited specimen sequentially, in which both of the interlayers and the Cr-C layer exhibit columnar structures. Oxidation of the Cr-C coated steel at high temperatures produces an oxide layer, Cr2O3, on the coating surface, and the sublayer of the original Cr-C coating has transformed into an orthorhombic Cr3C2 phase. In contrast to the as-deposited specimen, the transitional dual-phased nitride layer and the Cr3C2 coating layer in the oxidized specimens have equiaxed grain structures. Analytical results of the plan-view TEM indicate that oxidation of the chromium carbide-coated specimens begins with grain boundary or triple points of boundaries and the thickness and grain size of the oxide layer increases with the oxidation temperature. Elemental analyses of the Cr-C coatings at various temperatures using AES, EELS and XPS are discussed. The AES spectra reveals that the thickness of the oxide layer increases with the oxidation temperature and the activation energy of oxidation was estimated to be 2.10 eV. Furthermore, based on the chemical shifts of the elements in XPS and the white line ratio of the Cr-L23 in EELS spectra, the bonding states of Cr are confirmed to be Cr3C2 and Cr2O3 for the carbide layer and oxide layer, respectively.
|Appears in Collections:||材料科學與工程學系|
Show full item record
TAIR Related Article
Items in DSpace are protected by copyright, with all rights reserved, unless otherwise indicated.