Ž .Thin Solid Films 377]378 2000 788]792

Low pressure plasma spray coatings

Elizabeth J. Younga,U, Eli Mateevaa, John J. Moorea, Brajendra Mishraa,Michael Lochb

aDepartment of Metallurgical and Materials Engineering, ACSEL Research Center, Colorado School of Mines, Golden, CO 80401, USAbSulzer-Metco, Rigackerstrasse 16, Wohlen, CH-5610, Switzerland

Abstract

Ž .A new technique } low-pressure plasma spray LPPS } has been used for deposition of high quality Al O coatings on2 3aluminum substrates for many different applications. The initial results on the properties and structure of the LPPS alumina

Ž .coating are presented in this paper. Chemical compositional analysis using secondary neutral mass spectroscopy SNMS hasshown that the coatings are close to stoichiometric with an oxygen to aluminum ratio of 1.65. The cross-sectional transmissionelectron microscopy and X-ray diffraction investigations revealed that the structure of the coating is complex: layered splatstructure with splats consisting of a polycrystalline core embedded in an amorphous matrix. Initial dielectric strength measure-

Ž .ments show dielectric strengths of approximately 1000 kVrcm in some of the thinner 10]20 mm LPPS coatings, a valuesignificantly higher than in other thermal spray coatings. The dielectric strength of these LPPS coatings is related to themicrostructure as determined by electron microscopy imaging. The studies of the adhesion of the coating to the aluminum

Ž .substrate using scratch testing with a 5-mm ball indentor showed maximum adhesion critical loads for the 20-mm thick LPPScoatings. These adhesion data have been related to the microstructure of the coatings and the initial substrate preparation. Theeffect of different substrate preparation on improving adhesion is being investigated together with wear testing of the LPPScoatings. Q 2000 Elsevier Science B.V. All rights reserved.

Keywords: Plasma spray; Alumina; Dielectric strength; Scratchradhesion

1. Introduction

Al O coatings with high wear resistance, adhesion2 3and good electrical properties are being produced by anumber of different methods such as physical vapor

Ž . Ž .deposition PVD and chemical vapor deposition CVDtechniques, and plasma spray processes. PVD and CVDcoatings have high density and good quality. However,these techniques suffer from rather low depositionrates. Plasma sprayed coatings are produced at muchhigher rates but they generally exhibit much higherporosity and a definitive splat structure. Thus, the

U Corresponding author. Tel.: q1-303-384-2140; fax: q1-303-273-3057.

Ž .E-mail address: ejyoung@mines.edu E.J. Young .

major challenge is producing good quality coatings athigh rates to reduce production costs.

In the present work the possibility of depositing highquality alumina coatings by an alternative method isbeing investigated, i.e. low pressure plasma sprayingŽ .LPPS . The LPPS process can produce a 10-mm thickcoating over an area of 0.5 m2 in less than 1 min. Theefficiency of powder usage in this system is high, ap-proximately 80%. LPPS processing may be used inapplications where PVD coatings cannot cover largeareas or requires large processing costs, and wheretraditional thermal spray coatings have reached limita-tions based on porosity and thickness requirements.

The aim of this present work is to show that LPPSAl O coatings are of a high quality and, in particular2 3have good adhesion, wear resistance and dielectricstrength values. These properties may allow very im-

0040-6090r00r$ - see front matter Q 2000 Elsevier Science B.V. All rights reserved.Ž .PII: S 0 0 4 0 - 6 0 9 0 0 0 0 1 4 5 2 - 8

( )E.J. Young et al. r Thin Solid Films 377]378 2000 788]792 789

portant applications, for instance in the bearing indus-try for dielectric isolation, and in the automotive in-dustry for high-speed production of wear resistantelectrical contacts.

2. Experimental

2.1. Deposition

Sulzer-Metco Inc., Switzerland deposited the aluminacoatings by LPPS. A detailed description of the LPPS

w xdeposition procedure is given by Loch and Barbezat 1 .The coatings are produced in low-pressure conditionsin a vertical chamber. The vertical chamber designallows for changes in the distance between the sub-strate to the plasma plume carrying the molten coatingmaterial. Several powder-feeding hoppers are arrangedsuch that the powder feed rate for the a-aluminaprecursor is 200 grmin into the plasma nozzle.

2.2. Chemical analysis

Ž .Secondary neutral mass spectroscopy SNMS wasperformed to determine the stoichiometry of the coat-ings. The SNMS data for the bulk of the coatingsample confirmed that the oxygen to aluminum ratiowas 1.65, i.e. close to the theoretical ratio of 1.5 forAl O .2 3

2.3. Structural in¨estigations

The structure of the coatings was investigated usingŽ .X-ray diffraction XRD and cross-sectional transmis-

Ž .sion electron microscopy XTEM . XTEM was carriedout using a Philips CM200 microscope at 200 kV. TEMsamples were prepared by tripod polishing followed bybrief ion milling.

2.4. Scratch testing

The adhesion of the coatings was investigated using aTEER scratch tester, model ST-2200, with a 5-mm WCball. The loads applied were in the range of 5]100 Nwith a load rate of 100 Nrmin and a table speed of 10mmrmin. Scratch testing was performed on aluminacoatings deposited on substrates with different sub-strate surface preparations. The standard surfacepreparation is grit blasting in a small chamber with a

Ž .particle size range of 106]125 mm standard 120 grit ,at a pressure of 1]2 Pa. To compare the adhesion ofthe LPPS coatings as a function of substrate prepara-tion, 10-mm thick coatings were deposited on textur-ized substrates of the same aluminum alloy. Texturizinginvolves a process similar to shot peening with differ-ently sized media to produce the amount of textureapplied to the prepared substrate. The measurement of

roughness was done by laser profilometry. The r val-aues, the measurement calculated by determining a meanline through the scanned area and the average devia-tion from the mean line, for the texturized substrates isgiven along with the critical loads in Table 2.

2.5. Wear testing

A 120-mm LPPS coating was tested for wear resis-tance. The friction was compared and measured againsta 52100 steel ball with a 5.5-mm diameter. A 3-N loadwas applied; the approximate stress induced on thesample was 0.9 GPa. The relative humidity of the testwas held constant at 23%. The ball was run over thesample for 37 799 cycles before the test was terminated.

2.6. Dielectric strength

Dielectric strength testing was performed on a vari-ety of LPPS samples of different thickness. Three mea-surements were taken for each thickness of coatingusing a Veckman AC dielectric strength test set, modelPA70. The dielectric strength was calculated using thevoltage at breakdown of the coating divided by thethickness of the sample. The dielectric test was per-

Ž .formed in an oil bath, with 2.54 cm 1 inch copperelectrodes. The coatings were left on the substrates,where the contact of the electrode with the aluminumsubstrate acts as the back contact. The voltage increaserate applied to the samples was 1000 kVrs. The dielec-tric breakthrough points were examined using a scan-

Ž .ning electron microscope SEM .

3. Experimental results

3.1. Chemical analysis

The oxygen, aluminum and carbon concentrationsdepth profiles for a 10-mm thick coating determined bySNMS are shown in Fig. 1. As seen from Fig. 1, theoxygen and aluminum content remained constant overthe thickness of the coating. The oxygen to aluminumratio was 1.65, 10% higher than the theoretical stoi-

Ž .chiometry 1.5 of Al O .2 3

3.2. Structural in¨estigations

XRD measurements on the alumina coatings showedthat the coatings represent a mixture of amorphous,rhombohedral a-Al O and monoclinic g-Al O2 3 2 3phases, as typical for plasma-sprayed Al O coatings2 3w x2,3 . XTEM analysis allows us to determine the dis-tribution of these phases in the coatings. Fig. 2a,bprovide representative cross-section bright-field TEMimages of a 10-mm thick coating. The coatings exhib-ited a complex layered splat structure. The thickness of

( )E.J. Young et al. r Thin Solid Films 377]378 2000 788]792790

Fig. 1. SNMS analysis of a 10-mm LPPS sample to determine theoxygen to aluminum ratio in the coating material.

the individual splats ranged from 300 nm to 1 mm. Fig.2b provides an enlarged view of the splat microstruc-ture. The splat exhibited two distinctive areas of dif-ferent contrast: the darker contrast is from the splatcore and the lighter featureless contrast area is fromthe surrounding matrix. Electron diffraction of thesetwo regions confirms that the darker contrast stemsfrom regions with polycrystalline structure and thelighter contrast is from amorphous regions. Thus, thesplats consisted of a polycrystalline core embedded intoan amorphous matrix. The areas between the splats are

Ž .composed of amorphous material rather than voidsthat provided an even lighter contrast, which is anindication that these areas may have a higher oxygencontent. The porosity of the coatings determined fromthe XTEM images is below 1%, indicating a very densecoating. Fig. 3 is an XTEM image of the coatingrsub-strate interface. The interface between the coating andthe substrate is sharp and a thin amorphous interlayerthat follows the substrate surface morphology locallypromotes adhesion.

3.3. Scratch testing

Scratch testing was performed on alumina coatingsdeposited on substrates with different substrate surfacepreparation. The critical load values for a number ofalumina coatings of different thicknesses deposited ongrit-blasted substrates are given in Table 1. The criticalload for a 1-mm thick PVD coating produced by un-balanced magnetron sputtering is given in Table 1 forcomparison. The critical load data show that the LPPSalumina coatings with thickness of 20 mm or greaterhave good adhesion as compared to the critical load forthe PVD coating, using the 5 mm ball test. The scratch

testing results for coatings deposited on texturized sub-strates are shown in Table 2. The results indicate thatthe critical load of the coating is dependent on thedegree of texturizing, i.e. the density and depth of thepits formed by grit blasting not via the r value of theainitial substrate. The critical loads were greater for a10-mm coating when the roughened surface had adensely packed peak and trough morphology.

3.4. Wear testing

The sample tested showed no measurable wear anddid not have a final failure over the duration of thetest. The steel ball showed wear of 1800 mm before thetesting was terminated. The coefficient of friction mea-

Ž .Fig. 2. a XTEM image showing quasi-layer splat structure of theŽ .LPPS coating. b XTEM image showing close-up of splat internal

structure, with polycrystalline interior and surrounding amorphousmatrix.

( )E.J. Young et al. r Thin Solid Films 377]378 2000 788]792 791

Table 1Ž .Critical load values N for LPPS samples

Ž .Thickness mm 10 20 50 80 100 150 200 1Sample 3610 3611 3608 3612 3615 3616 3619 PVD

A 61.67 58.42 88.49 55.32 61.98 86.59 65.71B 48.02 78.57 62.46 68.49 59.37 80.56 63.81 45C 59.21 80.93 73.65 66.19 74.60 78.18 66.27

Average 56.30 72.64 74.87 63.33 65.32 81.77 65.26S.D. 7.28 12.37 13.06 7.04 8.15 4.34 1.29

Fig. 3. Interface area of coating and substrate, showing areas of localattachment of the coating to the substrate surface.

sured against the steel ball was 0.742 for the 37 799cycles of the test.

Table 2Ž .Critical load values N for 10 mm LPPS coatings substrates

produced by texturizing and standard grit blasting

Sample T10 T21 T4X T6X 120 gritr value 0.797 1.75 2.24 3.08 2.08a

1 42.42 25.75 41.76 46.04 61.672 54.68 54.75 49.45 48.81 48.023 43.11 49.13 41.76 51.11 59.21

Average 46.73 43.21 44.32 48.65 56.30S.D. 6.89 15.38 4.44 2.54 7.28

3.5. Dielectric strength

The break down voltages and the dielectric strengthvalues for alumina coatings with different thickness aregiven in Tables 3 and 4. Some of the 10-mm coatingsexhibited breakdown voltages within the charging re-gion of the equipment. These low breakdown voltagesmay be due to the large effect of the substrate mor-

Table 3Break-down voltages for the LPPS coatings, all voltages are in kV

Ž .Thickness mm 10 20 50 80 100 150 200Sample 3610 3611 3608 3612 3615 3616 3619

A 2.3 2.3 2.9 3.4 4.4 5.3 5.3B 2.2 2.2 1.9 3.7 4.6 4.0 6.3C ] 2.1 2.0 3.0 2.8 5.5 5.5

Average 2.20 2.27 3.37 3.93 4.93 5.70

Table 4Dielectric strength values for the LPPS coatings, values given in kVrcm

Ž .Thickness mm 10 20 50 80 100 150 200Sample 3610 3611 3608 3612 3615 3616 3619

A 2300 1150 580 425 440 353 265B 2200 1100 380 463 460 267 315C ] 1050 400 375 280 367 275

Average 2250 1100 453 421 393 329 285S.D. 71 50 110 44 99 54 26

( )E.J. Young et al. r Thin Solid Films 377]378 2000 788]792792

Fig. 4. Size of dielectric break-through points for LPPS samples.

phology on the final coating. Thinner areas may existwhere the coating is not adherent and the appliedvoltage reached the substrate without interacting withcoating material. The coatings with thicknesses of 20mm and greater produced measurable breakdown volt-ages. Coatings with thicknesses greater than 50 mmproduced dielectric strength values of 250]450 kVrcm.Values from the literature indicate dielectric strengthsof 1000 kVrcm for bulk pressed alumina and 3000

w xkVrcm for single crystal Al O 4 . Air plasma spray2 3coatings from some studies have produced dielectric

w xstrength values in the range of 100]200 kVrcm 5 .The 20-mm LPPS Al O coatings showed much higher2 3dielectric strength values than the thicker LPPS coat-ings, and were similar to the bulk pressed alumina

Ž .values Table 4 .The dielectric breakthrough points were measured

using SEM imaging. The sizes of the breakthroughŽpoints were within the same order of magnitude Fig.

.4 . This result shows that there is no apparent thicknesseffect at the point of breakdown for the samples.

4. Discussion

The structural analysis of alumina coatings depositedby LPPS confirm that the coatings have a complex splatstructure, with splats consisting of a polycrystalline coreembedded in an amorphous matrix. This complex typestructure is believed to result from the rapid cooling ofthe molten particles upon their impact with thealuminum substrate. The rapid cooling also produces

Ž .the mixed phase structure a-Al O and g-Al O in2 3 2 3the crystalline core of the splat. Another interestingfinding from the structural investigations is the very lowporosity of the coatings: -1%. The dielectric strengthvalues measured for the LPPS alumina coatings wereapproximately 1100 kVrcm for the 20-mm coatings and250]450 kVrcm for the LPPS coatings with thicknessranging from 50 to 200 mm. These values are signifi-cantly higher than the dielectric strength values for

APS alumina coatings that are generally of the order of100 kVrcm. It is postulated here that these very highdielectric strength values are due to the interplay of thecomplex morphology of the coatings and their very lowporosity. The intersplat regions of the LPPS coatingsare composed of amorphous material as opposed to airfilled pores in the case of thermally sprayed coatings.

The substrates are prepared by using 120 gritŽ .106]125 mm size range to promote adhesion of themolten alumina particles upon impact. Pits are formedin the substrate material, which affect the properties ofthe coatings depending on their thickness. Coatingsproduced thinner than 10 mm are not thick enough toovercome the substrate preparation. However, coatingson the order of 10 mm and thicker are very adherentand have good critical load values in addition to excel-lent wear properties.

5. Conclusions

The electrical properties of the coatings are affectedby the structural characteristics. The high dielectric

Ž .strength values 250]450 kVrcm appear to be depen-dent on the complex structure developed by the LPPSprocess. The porosity shown to be less than 1% doesnot appear to have an effect on the dielectric strength.These characteristics show a splat structure with com-plex crystalline material in an amorphous matrix.

The overall chemical composition of the coatings isclose to theoretical alumina. The critical load valuesfrom scratch testing indicate good adhesion of thecoatings on the substrate depending on the substratepreparation. The LPPS Al O coatings exhibit high2 3wear resistance, which will allow for use in many appli-cations.

Acknowledgements

This work is supported by Sulzer-Metco, Switzerlandand the ACSEL research center at the Colorado Schoolof Mines. The authors thank Mike Kochis of CoorsTekfor dielectric strength testing, Gary Doll of Timken forwear testing measurements, and Jerry Skoff of BadgerMetal Technologies for production of texturized sub-strates.

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w x4 C.T. Morse, G.J. Hill, Proceedings of the British CeramicSociety, Electrical and Magnetic Ceramics, 1970 p. 23.

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