Investigation of the thermophysical properties of the oxide layer formed by microarc oxidation on Al-Si alloy

The results of the experimental study of the thermophysical properties of the coating formed by microarc oxidation (MAO) on an aluminum-silicon alloy AK12D are presented. Тhe description of the research methodology, including the formation of the MAO-layer on the surface of laboratory samples and the study of their structure, composition and of thermophysical properties are given. The thermal conductivity of the coating was investigated by two methods: steady state and laser flash method. The tests were performed within the temperature range of 50-400° C. The obtained results showed that the coating have a rather low thermal conductivity (λ= 1.32...1.52 W/(m⋅K) at 100 оС). It is shown that a wealth of mullite (up to 60%) does not lower the thermal conductivity of the coating. It is assumed that the basic factors affecting the thermal conductivity of the oxide layer are small-scale porosity, crystallite sizes and amorphous phase.

Using of MAO-layers as thermal barrier coatings is greatly promising way of its application.The application of MAO-coatings to heated details of power equipment leads to reducing their heat stress enhances reliability and increases engine efficiency.This is very important for engine pistons, working at temperatures up to 400°C [10].
Application of MAO-coatings requires the knowledge of the basic thermophysical properties, such as specific heat capacity, thermal diffusivity and thermal conductivity.However, the thermophysical properties of MAO coatings are not completely investigated nowadays.Therefore, the reliable information about the thermophysical properties of MAO coating should be obtained experimentally.

Formulation of the problem
Investigation of the structure and thermophysical properties of the MAO-layers is a complex study.Therefore, the number of researches describing the thermophysical properties of MAO-layers is very insignificant nowadays.
There are a few papers, that describe the determination of the thermal conductivity of the MAO layer formed on a wrought aluminum alloy 6082 (with fraction Si of 0.7...1.3%) in a silicate-alkaline electrolyte [11][12][13].The thermal conductivity of the MAO-coating depends on its composition.It was revealed in [13] that the thermal conductivity of the mullite-rich coating decreases to λ=0.5±0.2W/(m⋅K).
All the thermophysical data were obtained for the MAO-coatings formed on the wrought aluminum alloy 6082.However, in the industry, alloys with a high silicon content (Al-Si alloys) are most common.Therefore, the study of the thermophysical properties of MAO-layers formed on Al-Si alloys is an actual challenge.This research is aimed to this problem.
The MAO-coating was formed on a disk, with diameter of 68 mm and thickness of 10 mm.The MAOprocess was performed in a silicate-alkaline electrolyte prepared with distilled water, Na2SiO3 (2.5 g/l) and KOH (2.5 g/l).The electrical parameters of the process were: the ratio of the anodic and cathodic current was kept constant Ia/Iс = 1, the average current density was j = 13±2 A/dm 2 , the pulse frequency was 50 Hz.The process took 90 min.The electrolyte temperature did not exceed 36°C.
Coated disk was cut into samples to measure its thickness, porosity and to analyze the structure.The sample for steady state method of thermal conductivity measurement had sizes of 45 mm x 20 mm x 10 mm.The sample for laser flash method had diameter 9.9 mm and thickness of 1.9 mm.
Thermophysical properties of the alloy AK12D (the heat capacity -Cpsb and the thermal conductivity -λsb) were investigated on samples with 9.9 mm diameter and 1.9 mm thick.The coefficient of linear thermal expansion (CLTE) -αsb was studied on the sample with diameter of 6.0 mm and length of 25 mm with dilatometer DIL-402 at temperature range of 50 -400 °C.
MAO-layer was prepared as the powder for studies of heat capacity and phase composition.

Thickness, porosity and density
Scanning electronic microscope (SEM) "JEOL JSM 6390" was used for the analysis thickness and porosity of oxide layer.Fraction of pores was calculated by the superposition of a square grid on the image of cross section structure of the coating [15].The SEM images obtained with backscattered electron mode.
Geometrical sizes and weight of the coated sample, coating thickness and density of substrate were used for the calculations of coating density.The density of aluminum alloy was measured with a special experiment.The samples were weighted using a Mettler Toledo XPE 26 microbalance, to ±2.5 µg precision.The geometrical sizes of the samples were measured using a LINKS digital measuring instrument with an accuracy of ±0.03 mm.

Phase composition
Phase composition was investigated by Rigaku Ultima IV X-ray diffractometer, using Cu-Kα radiation with 2Θ from 15 to 110°.MAO-layer prepared as a powder was used for XRD analysis as powders allow obtain more accurate results [16].Quantitative phase analyzis was carried out with Rietveld method, using the Maud software.

Heat capacity
Heat capacity of the MAO-layer within the temperature range of 50-400° C was measured using differential scanning calorimeter NETZSCH DSC 404F3 [17].
The measurements were made on the MAO-coating powder with a weight of 33.454 mg.A sapphire sample was used as the Cp standard.Cp measurement was carried out at 20 °С/min rate and isothermal hold at 400 °С during 3 min (the typical thermal program).Data were analyzed using the NETZSCH Proteus software.

Thermal conductivity
Steady state and laser flash methods both were used for thermal conductivity measurements.Coincidence of the results obtained with different methods increases reliability of the measurements.
Thermal diffusivity and conductivity of the coating and substrate both were determined with NETZSCH LFA 457 MicroFlash.This method determines thermal diffusivity with analysis of thermogram T(τ) of surface of the samples heated by laser flash W(τ) (Figure 1a).The shape of thermogram (Figure 1b) depends on thickness and thermal property of the sample layers.
Thermal diffusivity of the sample or the sample layer is calculated on the basis of the experimental thermogram.It is also possible to determine the heat capacity of the sample [18,19].The thermal diffusivity and thermal conductivity of the layers are calculated with Proteus LFA Analysis software, which contains methods for samples with one, two or three layers [19].The thermophysical properties of the AK12D alloy were determined on a sample without coating at the beginning of the experiment.The values of thermal diffusivity, heat capacity, CLTE and thermal conductivity were measured.
Thermal diffusivity both of coated and uncoated samples was measured.Thermal conductivity of MAOcoating was calculated with Proteus LFA Analysis software using thickness, density and Cp data of coating obtained in a special experiment.All the tests were performed within the temperature range of 50-400° C.
Thermal conductivity of MAO-coating was also measured with steady state method [20] within temperature range of 50-150°С.Scheme of experimental equipment is shown on Figure 2.This equipment contains heater, cooler, two heat conductors, thermal insulator and four tie studs.Differential scheme of thermocouples connections was used to measurement of temperature gradient into heat conductors.
A thin layer of Arctic MX-2 thermal compound was applied on top and bottom of MAO-coated sample.Then the sample was put into measurement equipment, heater was warmed up with electrical power, cooler connected with circulated cooling water.
The signals of thermocouples were registered when thermal state of the sample and heat conductors became steady.Thermal conductivity calculated as: where λc -thermal conductivity of MAO-coating; δc -MAO-coating thickness; q -heat flux; ∆Tsm -the difference between temperatures of heater and cooler surfaces connected with the sample; λsb -thermal conductivity of substrate, δsb -substrate thickness; rtthermal resistance between sample surface and heat conductor surface.Heat flux q and ∆Tsm were calculated by temperatures measured in heat conductors.λsb -was preliminary determined with the laser flash method.Thermal resistance rt was determined in special experiments during repeatable measurements on samples made of copper and aluminum.Value obtained in this experiment is rt = (1.8±0.2)•10 -5 (m 2 •K)/W.
Calibration of steady state experimental equipment was made by repeatable tests on the sample made of stainless steel.Thermal conductivity of this steel is exactly known [21].Uncertainties of thermal conductivity measurements with steady state method revealed in calibration experiment are less than 15% within temperature range of 50 -150°C.

Experimental results
The transverse section of the sample, on which the thickness and porosity were measured, is shown in Figure 3. Studies of the macrostructure of the oxide layer showed that the average thickness of the MAO-coating is 154.1±3.7 µm.The through and closed pores are present in the structure of the coating.The average porosity is 13.1 ± 1.6%.
The density of aluminum alloy AK12D determined in this study is 2679±4 kg/m 3 .The density of the MAOcoating is 2659±565 kg/m 3 .