comparison of the microstructural characteristics and electrical properties of thermally sprayed...

Comparison of the MicrostructuralCharacteristics and Electrical Properties

of Thermally Sprayed Al2O3 Coatingsfrom Aqueous Suspensions

and Feedstock PowdersFilofteia-Laura Toma, Lutz-Michael Berger, Stefan Scheitz, Stefan Langner, Conny Rodel,

Annegret Potthoff, Viktar Sauchuk, and Mihails Kusnezoff

(Submitted September 16, 2011; in revised form January 23, 2012)

In this work the microstructural characteristics and electrical insulating properties of thermally sprayedalumina coatings produced by suspension-HVOF (S-HVOF) and conventional HVOF spray processesare presented. The electrical resistance at different relative air humidity (RH) levels (from 6 to 97% RH)and values of dielectric strength were investigated by direct current electrical resistance measurements,electrochemical impedance spectroscopy, and dielectric breakdown tests. Relationships between elec-trical properties and coating characteristics are discussed. At low humidity levels (up to 40% RH) theelectrical resistivities of S-HVOF and HVOF coatings were on the same order of magnitude (1011 XÆm).At a very high humidity level (97% RH) the electrical resistivity values for the S-HVOF coatings were inthe range 107-1011 XÆm, up to five orders of magnitude higher than those recorded for the HVOF coating(orders of magnitude of 106 XÆm). The better electrical resistance stability of the suspension-sprayedAl2O3 coatings can be explained by their specific microstructure and retention of a higher content ofa-Al2O3. The dielectric strength Ed of suspension-sprayed coatings was found to be 19.5-26.8 kVÆmm21

for coating thicknesses ranging from 60 to 200 lm. These values were slightly lower than those obtainedfor conventional HVOF coatings (up to 32 kVÆmm21). However, it seemed that the dielectric strength ofconventionally sprayed coatings was more sensitive to the coating thickness (when compared with thevalues of Ed determined for S-HVOF coatings) and varied to a greater extent (up to 10 kVÆmm21) whenthe coating thickness varied in the range 100-200 lm.

Keywords Al2O3, dielectric strength, electrical resistivity,HVOF, microstructure, phases, suspension

1. Introduction

Thermally sprayed alumina coatings with suitabledielectric properties have been prepared and tested for the

manufacturing of electrostatic chucks, discharge devices,and insulating coatings for high-temperature heaters(Ref 1-5). Compared with sintered alumina, which consistssolely of the thermodynamically stable a-Al2O3 (corundum),thermally sprayed coatings contain mainly metastable phases(c- and d-alumina), even though all commonly used feed-stock spray powders consist of a-Al2O3. The properties of themetastable phases differ significantly from those of corun-dum; the stronger reaction of c-alumina with water or watervapour presumably has a negative effect on the dielectricproperties of the material. It is generally agreed that coatingsconsisting primarily of a-Al2O3 are desirable.

In a previous work by the authors (Ref 6) the electricalinsulating properties of alumina coatings produced byconventional APS and HVOF spray processes weredescribed. From the direct current (DC) electrical resistanceand electrochemical impedance spectroscopy (EIS) mea-surements it was shown that at low relative air humidity(RH) levels the values of electrical resistivity were around1011 XÆm for both the APS coatings and the HVOF aluminacoatings with a higher content of a-Al2O3. In humid envi-ronments the presence of a-Al2O3 is expected to play animportant role in the retention of electrical properties. Athigh humidity levels (>75% RH) the electrical resistancevalues obtained for HVOF alumina coatings were greater

This article is an invited paper selected from presentations at the2011 International Thermal Spray Conference and has beenexpanded from the original presentation. It is simultaneouslypublished in Thermal Spray 2011: Proceedings of the InternationalThermal Spray Conference, Hamburg, Germany, September 27-29, 2011, Basil R. Marple, Arvind Agarwal, Margaret M. Hyland,Yuk-Chiu Lau, Chang-Jiu Li, Rogerio S. Lima, and AndreMcDonald, Ed., ASM International, Materials Park, OH, 2011.

Filofteia-Laura Toma, Lutz-Michael Berger, Stefan Scheitz, andStefan Langner, Fraunhofer Institute for Material andBeam Technology (Fh-IWS), Winterbergstrasse 28, 01277Dresden, Germany; and Conny Rodel, Annegret Potthoff,Viktar Sauchuk, and Mihails Kusnezoff, Fraunhofer Institute forCeramic Technologies and Systems (Fh-IKTS), Winterbergstrasse28, 01277 Dresden, Germany. Contact e-mail: [email protected].

JTTEE5 21:480–488

DOI: 10.1007/s11666-012-9761-2

1059-9630/$19.00 � ASM International

480—Volume 21(3-4) June 2012 Journal of Thermal Spray Technology

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than those obtained for APS coatings. The electrical resis-tivity decreased by approximately five orders of magnitudein very humid environments (97% RH) for both types ofcoatings. Moreover, both coating types showed noinsulating behaviour (orders of magnitude of 106 XÆm). Asignificant degradation in electrical properties (decrease inresistivity of nearly six orders of magnitude) with anincrease in the air humidity (from 20 to 80% RH) waslikewise noticed for a-Al2O3 powders (Ref 7). The deteri-oration of insulation properties with increasing humiditycan be explained by the increase in surface electrical con-ductivity resulting from adsorption and accumulation ofwater monolayers on the oxide surface (Ref 8). Open poresand cracks in the coating microstructure should have adetrimental effect on the electrical properties. Thesecoating imperfections can generate transverse channels inwhich capillary condensation of water vapour can occur,reducing the coating�s electrical resistance.

Suspension thermal spraying and particularly suspen-sion-HVOF (S-HVOF) spraying allow dense aluminacoatings to be prepared from finely dispersed particles inaqueous or alcoholic suspensions (Ref 9-12). With appro-priate selection of both the raw powder for suspensionproduction and the spray parameters, thermal sprayingwith suspensions is an adequate method to producemechanically stable alumina coatings consisting mainly ofthe a phase without using additives such as ceramic oxides(Ref 13). The suspension characteristics, i.e., raw materialtype and purity, particle sizes in suspension, and solidcontent, have a significant influence on the retention ofa-Al2O3 (Ref 14). Suspension-sprayed alumina coatingsshow interesting mechanical and dry-sliding wear proper-ties (Ref 9, 11, 12). However, little information is knownabout the electrical insulating properties of such coatings.

In this work a comparative study on the electricalresistance at different air humidity levels and dielectricstrength values of alumina coatings produced by S-HVOFand HVOF processes was carried out. The coating char-acteristics of microstructure, crystalline phases, andmechanical properties are presented and the differences inelectrical properties due to the different spray processesand coating characteristics are discussed.

2. Experimental Procedures

2.1 Materials and Deposition Processes

A commercially available fine alumina powder char-acterised by a very high purity (a-Al2O3, >99.99% Bai-kowski, La Balme de Sillingy, France) was used toproduce an appropriate suspension for spraying. The rawpowder contained submicron-sized grains, as depicted inthe high-magnification SEM micrograph given in Fig. 1(a).The suspension was obtained by homogeneous dispersionof 35 wt.% solid in deionised water. Appropriate selectionof the suspension pH and addition of a small quantity ofan organic dispersion agent resulted in a highly stableaqueous suspension of low viscosity (around 3 mPaÆs). Theparticle size distribution (D90,3-D10,3) in suspensions

ranged from 0.4 to 3.5 lm and the average particle sizewas about 1.35 lm (Fig. 1b).

A commercial fused and crushed a-alumina powder(>99.7% purity, Ceram GmbH, Albbruck-Birndorf,Germany) with appropriate particle sizes for the HVOFprocess (�25 + 5 lm) was used to produce the conven-tionally sprayed coatings.

Both feedstock materials (suspension and powder)were sprayed with an HVOF TopGun (8-mm-diameternozzle, GTV mbH, Germany) using ethylene as the fuelgas. A modified combustion chamber allowing internalinjection of the suspension was adapted to the HVOF gun.Suspensions were fed from a pressurised vessel, asdescribed in a previous work by the authors (Ref 10). Themain spraying conditions for S-HVOF and HVOF pro-cesses are shown in Table 1. The spray parameters wereselected for the preparation of dense alumina coatings.S-HVOF coatings with average thicknesses ranging from60 to 200 lm were sprayed on grit-blasted copper plates(40 9 100 9 3 mm); HVOF coatings with average thick-nesses of 100 and 200 lm were obtained on mild steel(30 9 30 9 3 mm). For evaluation of electrical propertiesat different air humidity levels, selected S-HVOF and

Fig. 1 (a) High-magnification SEM micrograph of fine aluminapowder and (b) cumulative volume distribution of the particle insuspension

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HVOF coatings were sprayed on highly corrosion-resistant 1.4462 stainless steel plates (60 9 100 9 5.5 mm).All the substrates had been previously grit blasted withEKF 54 corundum prior to spraying.

2.2 Coating Characterization

The coatings� microstructures were examined by opticalmicroscopy and scanning electron microscopy on metal-lographically polished cross sections. The phase composi-tions were evaluated by x-ray diffraction using a D8(Advance Bruker AXS) diffractometer. Measurementswere carried out in the h-2h step scan mode using CuKaradiation and a step size of 0.05�. The phases were iden-tified using the Diffrac EVA software. The content ofcrystalline a-Al2O3 (Ca) in the coatings was estimatedusing the following equation:

Ca %ð Þ ¼ Aa 113ð ÞAa 113ð Þ þ 0:89 �Ac 113ð Þ � 100

The coating roughness parameters Ra, Rz, and Rmax weremeasured with a Mahr Perthometer (Mahr GmbH, Got-tingen, Germany) using a tracing length Lt = 17.5 mm(total measured length Lm = 12.5 mm) and a cut-offkc = 2.5 mm. The Vickers microhardness HV0.3 (load of2.94 N) was measured on the cross sections of 200-lm-thick coatings using an HP-Mikromat 1-HMV tester(Hegewald & Peschke Meß- und Pruftechnik GmbH,Nossen, Germany) with a load duration of 10 s. Young�smodulus was determined on the cross sections using aVickers micro-indenter (Shimadzu DUH-202 tester, Shi-madzu Scientific Instruments) with a loading rate of35 mNÆs�1 and a holding time of 10 s at the load of 200 g(1.96 N). The Young�s modulus values were extractedaccording to the Oliver-Pharr method (Ref 15).

2.3 Investigation of Electrical InsulationProperties

The insulation resistance and electrical resistivity of thecoatings at room temperature and at different air humiditylevels were investigated by DC measurements and thealternating current (AC) method, respectively. Dielectricbreakdown tests were performed to evaluate the dielectricbreakdown voltages (DBV) and dielectric strengths of thesprayed coatings.

The DC insulating resistance of the as-sprayed sampleswas measured using an Advantest R8340 Ultra-highResistance Meter (Advantest Corporation, Tokyo, Japan)

by applying DC voltages of 50, 100, and 200 V. Squareswith areas from 10 9 20 mm2 to 20 9 30 mm2, dependingon the sample geometries, were covered with silver pasteto ensure the electrical contact. During application of theDC voltage the electrical contact was placed at a distanceof about 2 mm from the edge of samples to avoid short-circuiting. For each voltage at least three measurementswere performed and the average values of insulatingresistance were determined.

The AC resistance/resistivity of the coatings wasdetermined using the EIS technique. The EIS measure-ments were performed with a Zahner IM6 ImpedanceAnalyser (Zahner-Elektrik GmbH & Co KG, Kronach,Germany) using a 2-electrode cell. A stainless steel elec-trode with a diameter of 3.5 cm was used as the counterelectrode, whereas the coating specimen played the role ofworking electrode. An alternating voltage with an ampli-tude of 5 mV at zero DC bias in the frequency range from1 MHz to 100 lHz was applied during the tests. EISmeasurements were performed on as-sprayed suspension-sprayed coatings, whereas the conventional coatings werefirst ground (at a Ra < 1 lm) to provide a good contactand to reduce the influence of surface roughness.

For investigation of the influence of the RH on theelectrical properties the samples were placed at roomtemperature in a desiccator and conditioned for longperiods (from 48 h up to several days depending on thecoating system) at different relative humidity levelsranging from 6% RH to 97% RH using saturated saltsolutions. The resistance values were then determined bymeans of DC and EIS measurements using the sameprocedures as described above.

The dielectric breakdown tests were performed onas-sprayed coatings, without any surface finishing or sealing,at room temperature and room humidity (40-50% RH)through application of high DC voltages using a PC6Ptestmeter (Sefelec GmbH, Ottersweier, Germany). Elec-trodes 5 mm in diameter were used as electrical contactsfor breakdown tests. The applied voltage was increasedlinearly at a rate of rise of 100 VÆs�1 from zero up to theoccurrence of flashover. A silver paste was applied to thecoating surface to provide a good electrical contact. Whenthe dielectric breakdown of the coating occurred the DBVwas measured and the dielectric strength (Ed) was deter-mined. The breakdown tests were applied on six or ninepoints on the coating surface of each sample and theaverage values of DBV and Ed were obtained.

3. Results

3.1 Microstructures and Phase Compositions

Both coating systems appeared to be dense in theoptical micrographs given in Fig. 2, although the S-HVOFcoatings showed a denser microstructure compared withtheir HVOF counterparts. High-magnification SEMmicrographs revealed a specific microstructure (Fig. 3aand b) for the suspension-sprayed coatings differing fromthat of the conventionally sprayed coatings (Fig. 3c and d).

Table 1 Main spray parameters

Parameter

Coating deposition

S-HVOF HVOF

C2H4/O2, LÆmin�1 75/230 90/270Spray distance, mm 80 150Relative torch scan velocity, mÆs�1 1.6 1.6Particle feed rate, gÆmin�1 33-35 27-30Scanning step size, mm 5 3Substrate cooling during spray Air jets Air jets


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Partially melted fine particles with elongated shapes (ofabout 5 lm in length and 150 nm in thickness), nearlyspherical lm-sized particles and clusters of agglomeratedtiny sub-lm-sized grains were embedded in the matrix ofwell-melted ones. The presence of small locally distrib-uted vertical cracks (up to several tens of lm in length)and closed pores (with sizes ranging from about 100 nmto a few lm) could be detected in the SEM micrographsof the S-HVOF alumina coatings. These small verticalcracks most probably resulted from the very high thermaland relaxation stresses generated during the sprayingprocess for the preparation of dense suspension-sprayedcoatings. The HVOF coatings were characterised by theclassic lamellar structure of thermally sprayed coatingsconsisting of well-melted and partially melted particles.The typical microstructural defects, inter- and intra-lamellar cracks, unmelted particles and pores, were alsoobserved.

The XRD patterns for the sprayed coatings are shownin Fig. 4. The HVOF coating contained the metastablec-Al2O3 phase (JCPDS card no. 10-0425) as the main phaseand a-Al2O3 (JCPDS card no. 46-1212). Traces of b-Al2O3

(JCPDS card no. 10-0414) already identified in the feed-stock powder (Ref 6) were also detected. In the S-HVOFalumina coating, the a-phase was the main crystallinephase. The content of a-phase was about 60% in the

S-HVOF coating, whereas only 30% could be retained inthe conventional HVOF coating.

3.2 Mechanical Properties

The microhardness values (750-850 HV0.3) and theYoung�s moduli (110-130 GPa; 1.96 N) obtained for theS-HVOF coatings were in the same range as those measuredfor conventional HVOF coatings (average microhardness:820 ± 30 HV0.3, Young�s modulus: 120 GPa (Ref 16)).Suspension spraying yielded smoother coating surfaces.Thus for S-HVOF coatings the values of Ra and Rz wereabout 2.4-3.5 and 15-22 lm, respectively, depending onthe coating thickness, compared with Ra of 4.1-4.8 lm andRz of 25-30 lm measured for conventional HVOF-sprayedcoatings. The roughness values obtained in this work forthe suspension coatings were significantly lower than thosereported by Bolelli et al. (Ref 9).

3.3 Electrical Resistance Measurements

3.3.1 DC Resistance. DC insulating resistances forcoatings with different thicknesses determined at roomtemperature in air at two different humidity levels (30-40%RH and 97% RH) are illustrated in Fig. 5. At comparablehumidity levels the absolute values of resistance for eachcoating system were almost independent of the coatingthickness. This was due to the fact that the size of theelectrode was significantly greater than the coating thick-ness; consequently, the contact surface area influenced theresistance more than the coating thickness did. At a lowhumidity level the insulating resistance values for theS-HVOF sprayed coatings were determined to be6.0 9 1010 X to 1.6 9 1011 X, in the same range as thoseobtained for conventional HVOF coatings (about1.0 9 1011 X). After 11 days of conditioning at 97% RHthe insulating resistances of the S-HVOF coatingsdecreased to 1.1-5.7 9 106 X (4-5 orders of magnitudelower). In contrast, the values of insulating resistance forHVOF coatings after only 48 h of conditioning at 97% RHwere about 3.2-3.8 9 105 X, about one order of magnitudelower than those measured for the S-HVOF coatings after11 days of conditioning at 97% RH. In very humid air theS-HVOF sprayed coatings were less sensitive to airhumidity than the conventional HVOF coatings were.

3.3.2 EIS. From the EIS measurements the contribu-tions of the resistive and capacitive components to theoverall total impedance in a large frequency domain as afunction of humidity could be estimated. At high fre-quencies the capacitive character (dielectric properties) ofthe impedance was predominant, whereas in the low-frequency domain the impedance was relatively independentof frequency, showing purely resistive behaviour (Fig. 6).The electrical resistance of the coatings could be deter-mined through simple R||C equivalent circuit model fit-ting. The coating resistance as a function of thickness canbe more conveniently expressed as the electrical resistivityq = (RÆS)/d, where R is the resistance (impedance), S isthe surface area of the stainless steel electrode, and d isthe coating thickness.

Fig. 2 Optical micrographs of alumina coatings obtained by (a)S-HVOF and (b) HVOF processes


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In this work only the results on electrical resistance(resistivity) are presented and discussed. The electricalresistivity behaviour of the coatings as a function of relative

Fig. 3 SEM micrographs of cross sections of thermally sprayed coatings at different magnifications: (a and b) S-HVOF and (c and d)HVOF

Fig. 4 XRD patterns of S-HVOF and HVOF alumina coatings

Fig. 5 DC insulating resistance as a function of coating thick-ness for S-HVOF and HVOF coatings in different humidityconditions (S-HVOF coatings conditioned in a desiccator at 97%RH for 11 days and HVOF coating for 48 h)


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humidity is shown in Fig. 7. The conventional HVOFcoatings were conditioned for 48 h, whereas the S-HVOFcoatings were tested after 48 and 86 h at each selectedhumidity level. In the case of conventional HVOF aluminacoatings the electrical resistance (resistivity) decreasedalmost linearly from 4.3 9 1010 X (4.0 9 1011 XÆm) at 6%RH to 9.5 9 105 X (8.8 9 106 XÆm) at 97% RH.

After 48 h of conditioning at 6% RH values of electricalresistance (resistivity) in the range 8.1 9 1010-1.1 9 1011 X(3.4 9 1011-1.8 9 1012 XÆm) were recorded for suspension-sprayed coatings with thicknesses from 60 to 200 lm.Comparable values were also obtained when S-HVOFcoatings were conditioned for 86 h at 6% RH. The thicknessof the suspension coating seemed to have little influence onthe electrical properties. In the humidity range from 6% RHto 75% RH the electrical properties did not change signif-icantly and values on the order of 1011 XÆm were recorded

for electrical resistivity. After 48 h of conditioning at 97%RH the electrical resistance (resistivity) of the S-HVOFcoatings varied from 3.0 9 108 X (4.9 9 109 XÆm) to1.7 9 1010 X (5.4 9 1010 XÆm), with the lower values beingobtained for the thinner coating (thickness of 60 lm). Theincrease in duration of sample conditioning in very humidair was associated with a slight decrease in electrical prop-erties, especially for the 60-lm-thick S-HVOF coating.Thus after 86 h of conditioning at 97% RH the resistanceand resistivity decreased to 9.4 9 107 X and 9.0 9 108 XÆm,respectively, about one order of magnitude lower than thevalues obtained after 48 h at the same humidity level.Nonetheless, these values were significantly higher thanthose recorded for the conventional coatings. At 97% RHthe electrical properties were found to be up to five orders ofmagnitude lower for HVOF coatings and up to three ordersof magnitude lower for S-HVOF coatings. Thereforeaccording to the statement of Ivers-Tiffee and von Munch(Ref 17) that for q > 108 XÆm the materials are consideredas insulators it can be confirmed that compared with theconventional HVOF coating, the suspension sprayed coat-ings retained their insulating behaviour for a longer timeeven in environments with high relative humidities.

3.4 Dielectric Breakdown Test

The average values of DBV and dielectric strength (Ed)as a function of the kind of coating and coating thicknessare shown in Fig. 8. In the case of suspension-sprayedcoatings the average values of DBV increased from 1.2 to4.0 kV when the coating thickness increased from 60 to200 lm. A breakdown voltage up to 4.5 kV was measuredfor a 200-lm-thick HVOF coating. By dividing thebreakdown voltage by the coating thickness the dielectricstrength can be obtained. For S-HVOF coatings withthicknesses ranging from 60 to 200 lm the average valuesof Ed were in the range 19.5-26.8 kVÆmm�1. In the case ofHVOF coatings Ed values were higher (up to 32 kVÆmm�1)than those obtained for S-HVOF coatings. However, itseemed that the dielectric strength of conventionallysprayed coatings was more sensitive to the coating thick-ness (when compared with the values of Ed determined forS-HVOF coatings) and varied to a greater extent (up to10 kVÆmm�1) when the coating thickness varied in therange 100-200 lm. The variation in dielectric strength withcoating thickness may provide additional informationabout the homogeneity and microstructural integrity of thecoating. A lower variation in the Ed in a given thicknessdomain (as observed for S-HVOF coatings) should indi-cate a more homogeneous coating structure.

4. Discussion of Electrical Properties

4.1 Electrical Resistance: Influence of Humidityand Coating Characteristics

DC resistance tests and EIS measurements showed thatunder conditions of lower air humidity (up to 40% RH)the electrical properties of the alumina coatings producedby both conventional HVOF and S-HVOF processes were

Fig. 6 Impedance as a function of frequency at different airhumidity levels for a 160-lm-thick S-HVOF Al2O3 coating(conditioned at each RH for 86 h)

Fig. 7 Influence of the relative humidity on electrical resistivityof S-HVOF and HVOF sprayed coatings (S-HVOF coatingsconditioned at different specific RH levels for 48 and 86 h andHVOF coating conditioned for 48 h)


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comparable. By exposing of the coatings to higherhumidity levels, especially >75% RH, drastic degradationof the electrical properties was seen (Fig. 7). Moreover,significant loss of resistivity, up to six orders of magnitude,was recorded for HVOF coatings after only 48 h ofexposure at 97% RH. S-HVOF coatings were less sensi-tive to humidity. Conditioning of suspension-sprayedcoatings for 48 and 86 h at 97% RH involved a decrease inelectrical properties of up to three orders of magnitude.After 11 days of conditioning at 97% RH the DC insula-tion resistance of the S-HVOF coatings decreased by up tofive orders of magnitude, but still remained one order ofmagnitude higher than the DC insulation resistance of theHVOF coatings determined after only 48 h of condition-ing (Fig. 5).

The deterioration of the electrical resistance withincreasing humidity could be explained by the increase inthe surface electrical conductivity resulting from theadsorption and accumulation of water monolayers on thesurface of the oxide (Ref 8) and the capillary condensation

of water in microstructural defects (cracks and openpores). As shown in the micrographs given in Fig. 3 sus-pension-sprayed coatings, in contrast to HVOF coatings,exhibited a specific microstructure with fewer interlamel-lar cracks and a significant amount of fine pores (Fig. 3aand b). The supposition that the S-HVOF coatings mainlycontained closed porosity and less open or interconnectedporosity was confirmed by the EIS measurements at highRH levels. S-HVOF coatings consisted mainly of a-Al2O3,which in humid environments is considered to be morestable than c-Al2O3, although the degradation of theelectrical resistance of a-Al2O3 powders with increasinghumidity is reported (Ref 7). It can be presumed that thesuperior electrical properties of suspension-sprayed coat-ings are the result of the combined effect of the specificmicrostructure and the retention of a high content ofa-Al2O3.

Besides, the presence of impurities can also affect theelectrical properties of the coatings. S-HVOF coatingswere produced from a powder characterised by a very highpurity (>99.99%), whereas HVOF coatings were sprayedwith a feedstock powder, which according to the productcertification data sheet contained of about 0.13 wt.%Na2O (see Table 1 in Ref 6). XRD analysis performed onHVOF coatings confirmed the presence of b-Al2O3, too.Beta-alumina is a good ionic conductor. It can be assumedthat in humid environments, the presence of Na+ ionsdecreases the electrical resistivity of HVOF coatings.

4.2 Dielectric Properties of the Coatings

The results of the dielectric breakdown tests indicatedslightly inferior dielectric strength Ed for the S-HVOFcoatings in comparison with the conventional HVOFdeposits (Fig. 8b). However, more in-depth analysis of theEd results showed that the changes in the dielectric strengthof S-HVOF coatings with changing coating thickness werelower than those obtained for HVOF coatings.

The mechanism of dielectric breakdown in the sprayedcoating was assumed to be more related to the coatingmicrostructure than to the phase composition. Turunenet al. (Ref 18) proposed a correlation between the coatingmicrostructure and nature of the pore architecture in thecoating and the dielectric behaviour. Cracks, especiallyvertical cracks in the coating microstructure, involve theformation of the critical failure path and consequentlythe coating will show dielectric breakdown. Moreover, thedielectric breakdown (dielectric strength) was consideredto occur through the heat/thermal mechanism (Ref 19) orelectrical discharge (Ref 20-22). When an electric field wasinduced in the coatings the electrical resistance increasedby a lesser amount than the thermal resistance of thecoating did; thus heat was rapidly generated through ionicconduction, but dissipation of the heat in the coatingvolume was slower. It can be assumed that the presence ofsmall pores and vertical cracks in the microstructure willenable the formation of local heating micro-zones becauseof the concentration of high electrical field densities,which will result in dielectric breakdown and coatingdamage.

Fig. 8 Variation of dielectric properties of suspension-sprayedcoatings as a function of coating thickness: (a) dielectric break-down voltage (DBV) and (b) dielectric strength Ed


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As shown in the SEM micrographs given in Fig. 3(a) and(b) suspension-sprayed coatings were densely structuredwith none of the lamella observed in the conventionalHVOF coating (Fig. 3c and d), but contained fine pores andlocally distributed vertical cracks. The electrical propertymeasurements indicated that the content, the shape and thedistribution of the microstructural defects (pores andcracks) played a decisive role in determining the electricalproperties of the coatings. It can be assumed that thesemicro-defects were �small� enough to avoid the percolationof the water vapour in the coating, but �big� enough to induceformation and concentration of high electrical field densi-ties. Nonetheless, according to the values of dielectricstrength determined for different coating thicknesses it canbe assumed that these microstructural defects were morehomogeneously distributed in the matrix of the S-HVOFcoating than in the HVOF coating structure.

Supplementary investigations will be carried out inorder to gain a better understanding of the relationshipbetween the dielectric properties and the pore networkarchitecture of the suspension-sprayed coatings.

5. Conclusions

A comparative study of the characteristics (micro-structure, phase composition, and mechanical properties)and electrical properties (insulating resistance, electricalresistivity, and dielectric strength) of Al2O3 coatings pro-duced by HVOF and S-HVOF processes is presented.

At low RH levels (<40% RH) the electrical resistivitywas on the order of 1011 XÆm for sprayed alumina coatingsobtained by both conventional HVOF and S-HVOF pro-cesses. At a very high humidity (97% RH) the electricalresistivity values for the S-HVOF coatings were in the range107-1011 XÆm, up to five orders of magnitude higher thanthose recorded for the HVOF coating (106 XÆm). The betterelectrical stability of S-HVOF coatings in highly humidenvironments can be explained by their specific micro-structure with finer pores and lower interconnected porosityand by the retention of a high a-Al2O3 content. However, atvery high humidities (97% RH) the electrical propertieswere found to degrade significantly (by up to 4-5 orders ofmagnitude) in these coatings after long-term exposure.

The dielectric strength of suspension-sprayed coatingswas found to be 19.5-26.8 kVÆmm�1 for coating thicknessesranging from 60 to 200 lm. It is supposed that the coatingmicrostructure, especially the presence of fine microstruc-tural defects, has a greater influence on the dielectricbreakdown than the phase composition does. However,further detailed study on the pore structures of the suspension-sprayed coatings and an understanding of the dielectricbreakdown mechanism are necessary for this to be proven.

Acknowledgment

The authors would like to thank their colleaguesat Fraunhofer IWS B. Wolf and S. Raschke for the

metallographic preparation of the samples and SEManalysis and S. Saaro for assistance with XRD analysis.The results presented here are part of the BmWi-InnonetResearch Project �INNOMODUL-Innovative Aufbau-technologie fur elektronische Hochleistungsmodule inFahrzeugen durch thermisches und kinetisches Spritzen�(INNOMODUL-Innovative technology for building ofhigh-performance electronic modules for vehicles bythermal and kinetic spraying), Project No. 16IN0695,funded by the Federal Ministry of Economics and Tech-nology, Germany.

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