Analysis of Al2O3 layers morphology and microstructure
The paper presents the characterization of obtaining Al2O3 oxide layers on aluminium AlMg2 alloy as a result of hard anodizing by the electrolytic method in a three-component electrolyte. The Al2O3 layers obtained on the AlMg2 alloy in the three-component SBS electrolyte were subjected to detailed m...
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Фізико-механічний інститут ім. Г.В. Карпенка НАН України
2010
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irk-123456789-317722012-03-18T12:26:58Z Analysis of Al2O3 layers morphology and microstructure Skoneczny, W. The paper presents the characterization of obtaining Al2O3 oxide layers on aluminium AlMg2 alloy as a result of hard anodizing by the electrolytic method in a three-component electrolyte. The Al2O3 layers obtained on the AlMg2 alloy in the three-component SBS electrolyte were subjected to detailed microstructural investigations (by means of a scanning electron microscope, SEM). By applying X-ray diffraction, examination of the obtained oxide layers phase compositions was carried out. It was found that the Al2O3 oxide layers obtained via hard anodizing in a three-component electrolyte are amorphous. The chemical composition of the Al2O3 layers is presented and compared to the results of stechiometric calculations for the Al2O3 layer. Surface morphologies of the obtained oxide layers are characterized and discussed in nano- and microscopic scales. The surface morphologies of the layers obtained have a significant influence on their properties, including their susceptibility to further modification (e.g. to incorporation of graphite), their wear resistance and the capacity for sorption of lubricants. Описано спосіб отримання оксидних покривів на основі Al2O3, сформованих на підкладці сплаву AlMg2 анодуванням у трикомпонентному електроліті. Мікроструктурні особливості шарів Al2O3 досліджено за допомогою сканівного електронного мікроскопа (SEM). Для вивчення фазового складу покривів використовували рентгенодифракційний аналіз. Виявлено, що оксидні покриви з Al2O3, отримані анодним оксидуванням у трикомпонентному електроліті, аморфні. Подано хімічний склад покривів та порівняно його з результатами стехіометричних обчислень. Проаналізовано морфологію поверхні оксидних покривів та обговорено їх поведінку на нано- та макрорівнях. Морфологія поверхні суттєво змінює інші властивості, зокрема, їх здатність до подальшої модифікації (включення графіту), зносотривкість та схильність до сорбції компонентів мастил. Описан способ получения оксидных покрытий на основе Al2O3, сформированных на подкладке сплава AlMg2 анодированием в трехкомпонентном электролите. Микроструктурные особенности слоев Al2O3 исследовано с помощью сканирующего электронного микроскопа (SEM). Для изучения фазового состава покрытий использован рентгенодифракионный анализ. Обнаружено, что оксидные покрытия из Al2O3, полученные анодным оксидированием в трехкомпонентном электролите, аморфны. Наведен химический состав покрытий и сравнено его с результатами стехиометрических расчетов. Проанализирована морфология поверхности оксидынх покрытий и обсуждено их поведение на нано- и макроуровнях. Морфология поверхности существенно изменяет другие свойства, в частности, их способность к дальнейшей модификации (включения графита), износостойкость и склонность к сорбции компонентов масла. 2010 Article Analysis of Al2O3 layers morphology and microstructure / W Skoneczny // Фізико-хімічна механіка матеріалів. — 2010. — Т. 46, № 2. — С. 130-135. — Бібліогр.: 8 назв. — англ. 0430-6252 http://dspace.nbuv.gov.ua/handle/123456789/31772 en Фізико-хімічна механіка матеріалів Фізико-механічний інститут ім. Г.В. Карпенка НАН України |
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The paper presents the characterization of obtaining Al2O3 oxide layers on aluminium AlMg2 alloy as a result of hard anodizing by the electrolytic method in a three-component electrolyte. The Al2O3 layers obtained on the AlMg2 alloy in the three-component SBS electrolyte were subjected to detailed microstructural investigations (by means of a scanning electron microscope, SEM). By applying X-ray diffraction, examination of the obtained oxide layers phase compositions was carried out. It was found that the Al2O3 oxide layers obtained via hard anodizing in a three-component electrolyte are amorphous. The chemical composition of the Al2O3 layers is presented and compared to the results of stechiometric calculations for the Al2O3 layer. Surface morphologies of the obtained oxide layers are characterized and discussed in nano- and microscopic scales. The surface morphologies of the layers obtained have a significant influence on their properties, including their susceptibility to further modification (e.g. to incorporation of graphite), their wear resistance and the capacity for sorption of lubricants. |
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Skoneczny, W. |
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Skoneczny, W. Analysis of Al2O3 layers morphology and microstructure Фізико-хімічна механіка матеріалів |
| author_facet |
Skoneczny, W. |
| author_sort |
Skoneczny, W. |
| title |
Analysis of Al2O3 layers morphology and microstructure |
| title_short |
Analysis of Al2O3 layers morphology and microstructure |
| title_full |
Analysis of Al2O3 layers morphology and microstructure |
| title_fullStr |
Analysis of Al2O3 layers morphology and microstructure |
| title_full_unstemmed |
Analysis of Al2O3 layers morphology and microstructure |
| title_sort |
analysis of al2o3 layers morphology and microstructure |
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Фізико-механічний інститут ім. Г.В. Карпенка НАН України |
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2010 |
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http://dspace.nbuv.gov.ua/handle/123456789/31772 |
| citation_txt |
Analysis of Al2O3 layers morphology and microstructure / W Skoneczny // Фізико-хімічна механіка матеріалів. — 2010. — Т. 46, № 2. — С. 130-135. — Бібліогр.: 8 назв. — англ. |
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Фізико-хімічна механіка матеріалів |
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1836627885103775744 |
| fulltext |
130
Ô³çèêî-õ³ì³÷íà ìåõàí³êà ìàòåð³àë³â. – 2010. – ¹ 2. – Physicochemical Mechanics of Materials
ANALYSIS OF Al2O3 LAYERS MORPHOLOGY
AND MICROSTRUCTURE
W. SKONECZNY
University of Silesia, Faculty of Computer Science and Materials Science, Poland
The paper presents the characterization of obtaining Al2O3 oxide layers on aluminium
AlMg2 alloy as a result of hard anodizing by the electrolytic method in a three-component
electrolyte. The Al2O3 layers obtained on the AlMg2 alloy in the three-component SBS
electrolyte were subjected to detailed microstructural investigations (by means of a
scanning electron microscope, SEM). By applying X-ray diffraction, examination of the
obtained oxide layers phase compositions was carried out. It was found that the Al2O3
oxide layers obtained via hard anodizing in a three-component electrolyte are amorphous.
The chemical composition of the Al2O3 layers is presented and compared to the results of
stechiometric calculations for the Al2O3 layer. Surface morphologies of the obtained oxide
layers are characterized and discussed in nano- and microscopic scales. The surface
morphologies of the layers obtained have a significant influence on their properties,
including their susceptibility to further modification (e.g. to incorporation of graphite),
their wear resistance and the capacity for sorption of lubricants.
Keywords: nano- and microstructure, surface layer Al2O3, surface characterization,
scanning electron microscopy (SEM), X-ray diffraction.
According to many authors [1–3] one of the most important research
directions in the field of new constructional materials in machine building are
materials resistant to operation at high temperatures (above 1000°C). Such
materials will be necessary for, e.g., car engines of new type, which must meet new
requirements concerning fuel consumption. Most probably, these will be ceramic
materials or materials covered with special coatings.
A characteristic feature of ceramic materials is their insignificant wear and a
low friction coefficient when co-working with other materials in the presence of a
lubricant. The most recent world trends in the machine building sector, in particular
with reference to piston machines, are heading to reduce their lubrication and
cooling. A question arises then, what the upper layer of a ceramic material should
be like in order to maintain the low wear and frictional resistance.
The Al2O3 layers belong to ceramic layers and, in the opinion of many authors
who deal with the problems of upper layers, the nearest era of future construction
materials will belong to ceramic materials. One of the most popular ceramic
materials is the aluminium oxide Al2O3 which, with the development of
technology, has found a number of new application areas over the past years.
Ceramic materials have physicochemical properties which predispose them to a
vast range of applications in structures working at high temperatures, i.e. in
combustion engines, gas turbines and piston compressors.
The possibility of covering aluminium and its alloys with oxide coatings has
resulted in an enhanced application of these materials, especially for:
Контактна особа: W. SKONECZNY, e-mail: ladyslaw.skoneczny@us.edu.pl
131
– components of couplings, transmissions, guides and slide ways;
– components in automatics and hydraulic controllers;
– rolling bearing races in a couple: steel – Al2O3;
– engine pistons and compressor cylinders sliding surfaces.
Experimental procedure. A new significant problem (due to the possibility
of shaping materials properties in a wide range) is the oxide layer on aluminium
obtained by means of hard anodizing. The basis for the hard anodizing of
aluminium is the fact that the coating is formed at the expense of mass decrement
of the substrate which turns into the Al2O3 compound.
The electrolytic processes of obtaining oxide layers on aluminium are
conducted in electrolyte solutions which partly dissolve the layer being formed.
Therefore, an essential amount of the electric energy consumed for the production
of an oxide layer on aluminium surface will not actually correspond to the
theoretical calculations.
The most widespread method so far has been anodic coating in a sulfuric or
oxalic acid at lowered temperatures, from –10°C to +5°C, depending on the type of
electrolyte. This required the erection of necessary installations to lower the
electrolyte temperature for surface treatment which was connected with high
capital expenditure. Therefore, researches on anodizing at elevated temperatures
have been conducted for years. The purpose of the researches is to apply such
electrolyte that would make it possible to obtain hard oxide layers at room
temperatures. Elimination of the electrolyte cooling stage would considerably
reduce the cost of oxide coatings production. It would be possible if hard anodizing
is conducted at temperatures of 20...40°C and higher.
At the same time coatings with better properties could be obtained owing to
the Al203 oxide phase transition at a temperature of 20°C. An increase in
temperature accompanying the oxidation process is conducive to etching
aluminium oxide fibers. In consequence, an oxide cell of a more regular (ideal)
structure is formed [4–7]. The increasing electrolyte temperature has also an effect
on the oxide coating porosity [1, 4, 8]. Porosity of the obtained oxide layers is of
major importance for their utilization for sliding cooperation with plastic materials.
The anodizing method developed by the author does not require cooling and
the process heat is used for controlling the properties of the oxide layers obtained.
Controlling of anodizing parameters allows, within some limits, programming the
selected functional properties of future upper layers [3–4]. According to the
method proposed, oxidation is conducted in a three-component water electrolyte.
Hard anodizing was conducted in a three-component electrolyte called SBS,
consisting of: sulfuric acid – H2SO4; oxalic acid – (COOH)2; succinic acid –
(CH2)2(COOH)2.
The process control factors are: current density – 2...4 A/dm2; temperature of
electrolyte – 293...313 K; oxidation time – 20...80 min.
Before the anodic oxidation process, the surfaces of specimens were ground
with abrasive papers of different gradation, always using the same technology, in
order to obtain an identical accuracy class for the precoat (irregularities height
Ra = 0.75 ± 0.15 µm).
Results and discussion. Microstructure and morphology of Al2O3 layers
surfaces. The structure of oxide layers is one of the main factors determining their
chemical, physical, surface and mechanical properties. Therefore the control of the
structure of oxide layers of Al and its alloys is very important for the electrolytic
132
method of producing some machine parts. In this connection the knowledge of the
formation mechanism of oxide layers obtained via the hard anodizing method is
essential.
Examination of the microstructure and morphology of Al2O3 oxide layers
obtained via hard anodizing was conducted using a scanning electron microscope,
Philips X130/ESEM/EDAX.
Based on the outcomes of the research on obtaining oxide layers on
aluminium in the first few seconds of electrodeposition, it can be affirmed that the
initially active places are roughness peaks of the substrate metal surface and crystal
ribs as well as the places with crystal lattice deformations and other defects of the
anode surface.
The effect of the oxide layer growth during the first several seconds of
anodizing is the formation of structure of dendrites. A conclusion can be drawn
based on this investigation that the size and quantity of substrate metal crystals, as
well as their form and orientation, i.e. their mutual arrangement, are of decisive
importance for obtaining the oxide layer final structure. The more are the substrate
metal crystals with a given growth direction in relation to the total number of
crystals, the higher is the orientation degree or the texture improvement degree in
the oxide layer obtained. So, it can be concluded that both the texture and the
crystal size are important factors, determining the properties of an oxide layer
obtained via hard anodizing.
The next stage of the layers
formation mechanism is transformation
of the dendritic structure into a co-
lumnar (fibrous) one. Such transforma-
tion takes place as a result of the applied
electric field which is directed from the
anode to the cathode and as a result of
another factor, i.e. the abstraction direc-
tion of the heat emitted very intensively
in the oxide layers growth regions. The
formed columnar (fibrous) structures,
oriented as a result of the electric field
influence and the heat abstraction direc-
tion from places of oxide layers growth (substrate), are presented in Fig. 1.
The structure of oxide layers depends, to a large degree, on the type and
concentration of electrolyte as well as on the conditions in which the hard
anodizing process is conducted. The anodic density of current applied during the
electrolytic process has a significant influence on the structure of anodic oxide
coatings. It influences the process speed which is strictly correlated with the
growth and size of crystals in the coating. Also, the temperature of electrolyte
affects the formation of coatings of a different grain size, which results from
changes in the conditions of secondary oxide dissolution.
The investigation results of the morphology of Al2O3 layers obtained via hard
anodizing are presented in Figs. 2.
Chemical and phase compositions of Al2O3 layers. The analysis of the
chemical composition of the Al2O3 layers obtained via the electrochemical method
in a three-component SBS electrolyte was conducted using a scanning electron
microscope Philips X130 with an EDS attachment. The analysis of the chemical
composition of the Al2O3 layer on a transverse microsection has shown that only in
Fig. 1. Columnar (fibrous) structure of the
Al2O3 layer.
133
the central zone, according to thickness, the chemical composition corresponded to
the stechiometric calculations. The chemical composition measurements carried
out in the middle part of the layer showed 52.93% aluminium content and a
47.07% oxygen content. According to the stechiometric calculations, the Al2O3
layer should have the following chemical composition: 52.92% Al and 47.08% of
oxygen.
Fig. 2. Nanopores (a–c), micropores (d–f) and developed morphology (g–i) of the Al2O3 layer
obtained via hard anodizing: j = 2 A/dm2, t = 60 min; b – j = 4 A/dm2, t = 40 min;
c – j = 4 A/dm2, t = 60 min.
Results of the chemical composition analysis of the Al2O3 layer in the middle
zone are presented in Fig. 3. According to the conducted research, when approaching
the Al substrate, the content of oxygen decreases and the Al content grows, whereas
when receding from the metal substrate, the oxygen content in the layer increases and
the Al content decreases. The research results regarding the changes in oxygen and
aluminium contents along the Al2O3 layer thickness are shown in Fig. 4.
Fig. 3. Fig. 4.
Fig. 3. Results of the Al2O3 layer chemical composition analysis.
Fig. 4. Change in oxygen and aluminium contents across the Al2O3 layer thickness: ♦ – O; ■ – Al.
134
A DRON-2 diffractometer was used for the X-ray phase analysis of the
obtained Al2O3 layers. The X-ray diffractogram of the Al2O3 layer obtained on a
crystalline AlMg2 aluminium alloy is presented in Fig. 5. All diffraction reflexes in
the figure originated from the metal substrate. The X-ray phase analysis has shown
that the obtained Al2O3 layers are amorphous.
Measurements of the Al2O3 layers microhardness done on transverse
microsections have shown that as the oxygen and aluminium contents change, the
microhardness (µHV) decreases. Examination of the layers microhardness was
conducted using a Neophot 2 microscope with a Hanemann’s attachment
(microhardness tester).
The results of distribution of the Al2O3 layer microhardness according to the
thickness are presented in Fig. 6.
Fig. 7. Fig. 8.
Fig. 7. X–ray diffraction pattern of oxide layer.
Fig. 8. Distribution of the Al2O3 layer microhardness according to the thickness: ♦ – µHV.
CONCLUSIONS
On the basis of the researches on the morphology of the nano- and micro-
structure, the chemical composition and phase composition of Al2O3 layers obtained
on the AlMg2 aluminium alloy via hard anodizing in a three-component SBS elect-
rolyte, as well as using the theoretical analysis results, it can be concluded that:
– the Al2O3 layers obtained on the AlMg2 alloy as a result of hard anodizing
are completely amorphous;
– the Al2O3 layers obtained as a result of hard anodizing have a fibrous
structure of amorphous layers;
– the Al2O3 layers obtained in the SBS electrolyte have a chemical
composition similar to the stechiometric calculations for Al2O3;
– the Al2O3 layers have a developed surface morphology being the result of a
fibrous structure (the effect of which are the nanopores formed at the contact points
of aluminium oxide fibers) and of the secondary dissolution of electrolyte as well
as energetic interferences (micropores being the effect).
РЕЗЮМЕ. Описано спосіб отримання оксидних покривів на основі Al2O3, сформова-
них на підкладці сплаву AlMg2 анодуванням у трикомпонентному електроліті. Мікрост-
руктурні особливості шарів Al2O3 досліджено за допомогою сканівного електронного мік-
роскопа (SEM). Для вивчення фазового складу покривів використовували рентгенодиф-
ракційний аналіз. Виявлено, що оксидні покриви з Al2O3, отримані анодним оксидуван-
ням у трикомпонентному електроліті, аморфні. Подано хімічний склад покривів та порів-
няно його з результатами стехіометричних обчислень. Проаналізовано морфологію по-
верхні оксидних покривів та обговорено їх поведінку на нано- та макрорівнях. Морфоло-
135
гія поверхні суттєво змінює інші властивості, зокрема, їх здатність до подальшої модифі-
кації (включення графіту), зносотривкість та схильність до сорбції компонентів мастил.
РЕЗЮМЕ. Описан способ получения оксидных покрытий на основе Al2O3, сформи-
рованных на подкладке сплава AlMg2 анодированием в трехкомпонентном электролите.
Микроструктурные особенности слоев Al2O3 исследовано с помощью сканирующего
электронного микроскопа (SEM). Для изучения фазового состава покрытий использован
рентгенодифракионный анализ. Обнаружено, что оксидные покрытия из Al2O3, полу-
ченные анодным оксидированием в трехкомпонентном электролите, аморфны. Наведен
химический состав покрытий и сравнено его с результатами стехиометрических расчетов.
Проанализирована морфология поверхности оксидынх покрытий и обсуждено их пове-
дение на нано- и макроуровнях. Морфология поверхности существенно изменяет другие
свойства, в частности, их способность к дальнейшей модификации (включения графита),
износостойкость и склонность к сорбции компонентов масла.
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Received 30.04.2008
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