Infrared determination of α-alumina in plasma sprayed λ-alumina

Materials

Chemistry

INFRARED

M-1.

and Physics,

10 (1984)

DETERMINATION

BARATON,

Laboratoire

OF

413

a-ALUMINA

IN PLASMA

SPRAYED

II-ALUMINA

P.QUINTARD de Spectroscopic

P. CHAGNON Equipe Thermodynamique

Vibrationnelle

et Plasma

Centre d'Etudes et de Recherches Facultk des Sciences, 123 avenue F-87060 Limoges Cedex (France) Received

413-424

4 October

1983;

320

Ceramiques (L-A. Albert Thomas

accepted

25 November

du

CNRS)

1983.

ABSTRACT

Plasma sprayed y-alumina often contains a few percent of the a-phase, depending on spraying conditions. The concentration of the a-phase, and hence the properties of the coatings, are linked to nearly forty parameters. We have developed a sensitive infrared method for the determination of the a-phase in y-alumina and selected the four most important parameters: particle size, power input of the gun, substrate velocity and cooling rate of the target. Infrared results are correlated with different mechanical properties of the deposit: texture of the polycrystalline surface .. . . porosity, friction coefficient, according to a fractionnal factorial design.

INTRODUCTION Among the P.V.D., nearly

various

etc)

the

40 X of

methods

plasma

all

spectacular

(Fig.

thod

[3,4,5]

preparing

for

providing

can be sprayed oxides

perties

mal barrier

In this

a quite

is

growth

rate

spraying

coatings,

(thermal

employed

up to

(oxides,

carbides,

borides,

position:

nearly

of

alumina,

wear

They are

are

and/or

also

used

spraying,

the present

the

last

C.V.D., time

ten years

now a well

several

can be melted.

constant

at

over is

they

when preparing coatings.

Its

Plasma

thick

that

coatings

process

[1,2].

and particularly [6,7]

industrial

1)

by plasma

represent

materials,

obtaining

applications.

been

materials,

for

spraying

millimeters

nitrides,

of

corrosion

diverse that

silicides,

etc), of

and mechanical

resistant

in optical,

me-

materials

50 %. The advantages thermal

has

established

Among the various

their

in

deposits

electrical

these

pro-

or

ther-

and nuclear

applications. work we develop

traces

of

a-alumina

us to

establish

0254-0584/84/$3.00

an infrared

in plasma

sprayed

the main spraying

spectroscopic y-alumina.

parameters

method An analysis

which

promote

for of the

determining variance

allows

achievement

0 Elsevier Sequoia/Printed in The Netherlands

414

160 1 140

100..

80.

60

0

of

58-61

62-65

pure

y-alumina.

to measure of

66-69

the

70-73

of

of

systematic price

the

infrared

the mechanical

the plasma

PLASMA

78-82

A factorial

the prohibitive

results

74-77

Fig-l. Number of published works on deposit processes [1,2], shaded parts display the growth of interest in plasma spraying techniques.

design

was used

effects

of

and the

time

forty

of

of

the

as it

macroscopic

required

determination

properties

here,

for

the a/y

deposited

is

quite

parameters

a complete

ratio

are

material

impossible on account

experiment.

correlated

The

with

some

and optimum values

of

. Tfie material

to be

parameters.

SPRAY

PROCESS

Apparatus The diagram deposited, bilized

of

the plasma

150 meters

arc.

per

second,

and quenched.

copper

anode,

stabilizes flux

of

hot

gases

while

their

10’

that

The ring-shaped a uniform and

of

is

the

melted

is

in

was built

(D4)

rotated

The plasma hydrogen,

gas

giving

where

is

an arc

they

in our

can withstand

a thermal (D5) up of

voltage

barrier

rapidly

of of

rapidly has a

Water

which

in

(Dl)

a thermal stops

the

the

droplets

layers. order

75 NL mn-’

65 V.

are

cools

successive speed

a mixture

sta-

laboratory,

which

and

water

up to about

and thorium.

at an adjustable (D2)

a D.C.

tungsten

is

stacking

in

and accelerated

which

electrodes,

the

at D3,

substrate,

made of

jet

Fig.2

on the

damage the substrate, resulting

shown in

as a powder

gun,

Wcmm2. The air

target

deposit.

15 NL mn-l

The plasma

is

tieated,

and splashed

and cools

could

impact,

are

the cathode

the arc up to

introduced

Particles

cooled

after

is

a-alumina, blown

equipment

to obtain of

argon

415

substrate &

Fig.2. Diagram of the plasma set up. D1 : Water cooling system for stabilizing the gun - D2 : Plasma gas IAr + Hz) - 03 : Powder feeding system - D4 : Thermal barrier : air jet for stopping hot gases - 05 : Cooling of the deposit.

Main parameters Taking into account the theoretical, practical and empirical knowledge of the plasma spraying process which has been obtained in our laboratory, we selected, among the forty parameters which must be controlled, the four which have the most influence in the a-AlzOx/ y-Al203 ratio [8,9,21,22]. They are: X(1) The maximum of the granulometry repartition curve X(2) The electric power supplied to the plasma torch X(3) The substrate velocity X(4) The cooling air flow. For each parameter we took five levels, arranged in a systematic pattern as reported in Table I. A complete factorial design would have needed 625 expensive experiments, so we used a fractional factorial design in a latin square. It allowed us to study four factors with five levels using only 25 judiciously choosen experiments [12,13,14] in such a way that for each level of one factor, five values are obtained with superimposition of variable values of the other factors according to the planned experiments in Table II. i is the number of the experiment, X(J) the level of the parameter J in Table I.

Table I. The five levels (J) of the four most meaningful for checking the statistical design.

parameters

(i) used

416 Table II. Planning of experiments in the fractional design in Latin square. X(5) and ~(6) values allow obtention of the background (or noise) from either uncontrolled parameters or from unconstant environmental conditions.

INFRARED

MEASUREMENTS

The y-phase Its

infrared

(Fig.3)

of

alumina

spectrum

characteristic

10

12.5

has a metastable

consists of

spine1

essentially

a disordered

16.f 3

25

of phase

structure

a very broad [15]

with

vacant

band

(900-300

sites. cm-‘)

.

50 f_lm

h1 // YJ 8QQ

e

0

’

,

2 IQ cm-’

Fig.3. Infrared absorption spectra of pure and finely y-alumina, and a mixture of Cr/y = 10% by weight

ground a-alumina

and

417 The a-phase

has an hexagonal

structure

formula units in the rhomboedral [16] yields

six infrared

degenerate

primitive

active modes

fine powder

transmission

mzasurements

Absorption

measurements

two A1203

cell. The factor group analysis

: two of the A2u type and four doubly

modes of type E . The six infrared

590(F) - 638(F) - 725(F,vb)

studies

of space group D$d with

active

fundamentals

at (Fig.3) 385(F) - 448(F)

appear

in

- 498(m) -

cm-'.

on traces of a-Al203

from the feeding powder

of the y + a phase change, were conducted

Spectral

resolution

was 2 0.5 cm-l, while

adjusted

to avoid any deformation

and

on a P.E. 225 spectrometer.

the scan speed and time constant were

of the band shape or variations

on the posi-

tion of the maximum. The finely ground deposit was smeared ethylene-gloved particles using

finger. Electron

less than 3 urn in diameter.

the classical

For the different by grinding,

pellet

technique,

as it has been shown

hand, we never pelleting

bands

The amount

secondary

RESULTS

of a

in plasma

intensities

[17,18] that there is a progressive and an increase

of the particles

transformation

sprayed y-alumina

of a to y (Fig.3)

reduction

in the intensity

decreases.

On the other

as a result of grinding

was determined

. It

[19]

is of sufficient

and

from the ratio

has been found that

accuray with regard

. From a calibration

[20]

that the four choosen

and for each of the 25 experiments the physical

variables

sity, friction and

X-ray

to other

curve we can detect

parameters

an

(Table II) we measured

were independent

on the samples M(i),

and texture of the polycrystalline

intensity,

poro-

surface.

spectroscopy

results are of particular

spectroscopy

However

interest

From a preliminary

can yield directly

as they are used in calibration

the percentage

for low values of a/y X-ray analysis

and only the strongest

the a/y ratio the Y-phase).

deposition

: a/y ratio, X-ray diffraction,

all other measurements.

absorption

in y-alumina.

such as

coefficient

and X-ray

interpreting

sensitive

granulometry

DISCUSSION

We postulated

infrared

(2 lmg in 800 mg).

as low as 0.5 % by weight.

AND

Infrared

with CsI as the matrix

and scattering,

any phase

causes of errors

Infrared

the powder was checked

on a plate.

this simple and quick method

a/y amount

that the layer was made of

For comparison,

as the diameter

observed

or smearing

of absorption

on a CsI plate using poly-

showed

spectra we took great care to obtain a constant

of light losses by reflection of the absorption

by friction

microscopy

(116-024-113

curve, of a-alumina

was insufficiently

lines could allow us to reach the evolution

and 104 for the a-phase

and 440-400-222

of

and 311 for

0.

0.

0:

2 11345!2Z(45

3.

1

..-..5

4‘

123151

a

3

1 x3

6

X(3) (substrate velocity) for different Fig.4. Infrared a/y(%) plotted against levels of X(l) (diameter of the feeding powder) Fig.5. X ray a/v (arbitrary units) as a function of X(3) for increasing X(1)

Comparison

of

and 5 on which meters

X(1)

results

is

we plotted

(diameter

of

example , on the abscissa X(3)

= 2 (velocity number

allows

us to obtain

14. This

same time

to display

different

levels

on curves

aberrant

results

Such results or in that

are

of

no17

by a simultaneous

percent

feeding

powder)

we plot

for

type

the

of

changes X(1).

M(f)

to be detected due either failure

and 24 arose

to

will

with

of

of

M(f)

of

of

from a defect

equipment. in powder

to

on,

as it

and at

is

also I.R.

environmental For feeding

example,

the

X(3)

observed

analysis

process,

in uncontrolled

For

the parameter

to X(3)

method

measuring

the plasma

later

para-

= 39nm) and

corresponding

used

Fig.4 of

velocity).

X( 1) on M(i)

variation

of

levels

= 3 (diameter

be widely

This

either

a change in

(substrate

X(1)

the effect

and 5). for

against

of measurements

The relationship (Fig.4

examination

infrared)

and X(3)

result

about of

for

the value

graphing

information

P-Q-R-S-T

an instantaneous result

(in

= 15 m s-l)

sample

cally

facilitated a/v

for

graphiallows

or X-ray. parameters we found

of the plasma

419

installation. Departures from the P-Q-R-S-T lines might originate either from random distribution of each measurement (experimentalerror) 1e.g X(1) = 1 and X(3) = 1,2,3,4,5 by X-ray) or from the effects of parameters that have not been taken into account (X(2), X(4), X(5)1. Nevertheless, a good correlation coefficient between infrared and X-ray results is found (0.986), whether or not experiments number 17 and 24 are included. Hence, these two ewperiments can be rejected only on the basis of other physical analyses. A more accurate analysis could be made by running a second series of 25 experiments, which can help to exclude aberrant results. Or, again, one could carry

.

out several M(1) for each sample (e.g. by I.R. or X-ray) in order to minimize random valuds by using their means. This procedure would only be possible if the sample were not destroyed by the measuring technique. If none of these procedures can be used, one can replot the aberrant measurements by fitting them to the curves according to their most probable values. In our case a statistical treatment of o/v values could only refine the results and corroborate the relationships such as these represented by P-Q-R-S. We postulated the position of the T line because averages of multiple infrared measurements, even though they minimized the scattering,couldnot be arranged on straight line. We must note at this point that the high level of granulometry surely did not permit a complete melting of the particles. This problem will be discussed further.

- 1 .Q d

L

_-_ - -1.9

Fig.6.

'

i

Deviations

same parameter

--

-x45

i

;

from the general

mean,

of means of five levels

of the

420 Analysis

of

variance

A classical

statistical

The difference

between

same parameter

is plotted

analysis

of an experimental

against

the five levels of this parameter.

ted. The dotted lines are the 'background sent secondary

effects

Granulometry

velocity

in this analysis,

of alumina particles

of the feeding powder.

This can be

theoretical

[7-10, 21,223

up to the melting

of the plasma,

at which point (=106KsA).

torch, the melting

point

analysis

the droplets Regardless

temperature

variance,

if one considers

29 kW, the respective

the weak effects

in the plasma.

considering

before

from the background to display

(Fig.6). In this

of the power variation

from the cl-feeding powder.

on the resifrom 20 to

solidification.

Hence

ched. For example,

Table III points out that for the range of diameters on the axis, the heat exchange

are respectively

we also can explain

multiplied

some reasons

on the

are not sufficiently

dered the velocity particles

on

are accom-

and lower degree of flattening these droplets

resi-

and the depo-

Moreover,

larger diameters

spread on the target,

so the

a significant

For example when X(2) varies

and velocities

out

power input to the of the particle;

the heart of the large particles

o-phase

the droplets which

pagned by lower viscosity substrate

point

moves

times are 520 and 560)s. For the largest diameters,

non-transformed

and

impinge upon the target and are

is reached at the surface

dence times are too short to melt sit encloses

at this melting

as the particle

of the electrical

in power level was not adequate

dence time of particles

of the temperature

is

. Particle temperature in the plasma

(2326 K), remains

effect of the power could not be extracted case the variation

repre-

deposition

during its flight in the jet ('5OOl1s) and decreases

quickly quenched

which

and the rate of Corundum

diameter

on the basis of a previous

arc increases

noise'

For example was calcula-

of the other parameters.

is the leading parameter

increased with increased

explained

is given in Fig.6.

1 (Fig.6a) the mean of results of experiments n"1,2,3,4,5

at X(1) =

rapidly

design

the general mean and the means of the five levels of the

surface

quen-

consi-

and the volume of

by a factor 0.4-11 and 37. At this point

for the dispersion

of results

for the T line

Table III. Half height width (HHW). and width (W) of the granulometry repartition curve with 75% particles; V = axial alumina particle velocities at 50 mm from the torch nozzle with a 28 kW power in a N2-H2 plasma jet.

421 on Fig.4 and 5. This dispersion the granulometry gradients

repartition

as indicated

large particlesin

might

curve

come from a large width

(Table III) and for high radial

in the cartography

an Ar-Ha plasma.

speed on injection were adjusted

With increasing

of the velocity

Such effects

substrate y-phase

has,

substrate cooling

more rapidly, therefore

temperature

(D5), models

ting during plasma

TO determine pure y-phase,

speed of rotation

of

of the

target

time and transfers

the quenching.

and divergent-

X(3) (Fig.6c) has a the deposit its heat

stays

to the

The high temperature

, a shorter time in which to begin its a-transition. was always

inferior

of the temperature

spraying

conditions

parameter

in the alumina plasma

to be considered

for the principal

to display

them graphically.

The

to 330 K so that with an efficient

distribution

never needed

it is of interest

(1.K.X) against

., ./y (IRX

thus increasing

the optimal

levels by presenting

symetric

of the substrate

in front of the torch flame for a shorter

and on-axis

and temperature

might arise even if particle

to give an homogenous

less jet (40 to 50 m s-l). The velocity weak effect.

at half heigtiof

changes

For example

X(3) for different

[Zl]

parameters

of y-Al203

coa-

. yielding

the

for different

on Fig.7 we plotted

X(l) and we obtained

a/y

the sensitivity

1

5.

Fig.7. Effect of parametersX(1) and X/3) on O./v alumina ; B is for the influence of X(l) alone. Fig.8. An example of non correlation of X(2) with a/y (however a/y correlates with X(1) (BJ

422 of a/y to variations of X(3). Typically the results are of 1% when X(3) increa ses by a factor of ten. Sometimes it is difficult to display the influence of one parameter, as in Fig.8, where the effects of changes in electrical power input

are not apparent. In this case the test is not powerful for reasons we

mentionned before and the relation a/y (IR) = f (Xl) can be clearly shown Fig.8B and only the means of five levels of X(2) are meaningful. CORRELATIONS

WITH

MECHANICAL

PROPERTIES

The values of a/y(%) from infrared measurements can be correlated to different mechanical properties of the deposit, some of which are plotted in Fig.9 and 10. The friction coefficientwas determined by a dynamical method; the firmed sample is in contact with a rotating steel ring on which a strength is applied. We measure the tangential strength T. The ratio T/N is an appraisal of this coefficient, either dry (Fig.9A) or wet (Fig.lOB).Here the crosses are related to mean values of a set of five levels for each parameter. For the dry sliding coefficient we added the influence of the following main parameters, cooling flow, velocity of the target, and granulometry, in order of decreasing importance.

A

0.25 10

0.40

cl30 12

B

20

Fig. 9. Dry friction coefficient (A) and texture ofpolycrystalline Fig. 10. Wet friction (A) coefficient and porosity (BJ

surface

.

(B)

423 Changes in X-ray intensities for a given crystalline plane (e.g. 311) related to ASTM standards give indications on the polycrystalline orientation of the plasma deposit (Fig.9B). This is, of course, related to o/y

and to friction

coefficients [23] as well as interelated to quenching speed. The values for porosity are obtained from a liquid impregnation technique (Fig.lOB). All these results for which we give only some examples here will be discussed elsewhere

CONCLUSION The quality of a plasma-sprayedy-alumina coating is a complex function of the sprayed material and of macroscopic factors in the operating conditions. The optimization of the working conditions of a plasma spraying device can be achieved with a minimum of experimental adjustments using planned experiments and an incomplete factorial design. The experimental programme leads to the following questions: which parameters affect the cc/yalumina ratio and how and why they affect the responses on M(T) within the range of variable studies. Infrared absorption spectrometry is of fruitful assistance in setting up the best conditions for the plasma arc in order to obtain a minimum a-phase in the y-sprayed alumina. The amount of cc/vcan be correlated with mechanical properties and allows prediction of these properties.

REFERENCES

6 7 8 9 10 11 12 13 14 15 16

CHAGNON P. , Thesis, Limoges, (1982) (with 310 references) CHAGNON P., and FAUCHAIS P. to be published KASSABJI F., and FAUCHAIS P., Rev. Phys. Appl. 16, 549 (1981) FAUCHAIS P., and RAKOWITZ J., J. de Phys. F, =,289 (1979) FAUCHAIS P., and BOULOS M., Heat and Mass Transfer under Thermal Plasma conditions, (to published) in Advances in Heat Transfer , John Wiley, N.Y. (1984) BOCH P., FARGEOT D., GAULT C., PLATON F., Rev. Int. Htes Temp. 2,85 (1981) 425 (1981) VARDELLE M., BESSON J.L. ,FAUCHAISP.,Rev. Phys. Appl. 2, VARDELLE M., Thesis, Limoges (1980) ZOLTOWSKI P.,Rev. Int. Htes Temp. 5, 253 (1968) VARDELLE M., and BESSON J.L., Ceramics InternationalI,48 (1981) MCPHERSON R., J. Materials Science,& 851 (1973) MANDEL J,, The Statistical analysis of experimental data, John Wiley, N.Y. (1964) DIXON W.J., and MASSEY F.J., Introduction to statistical analysis, McGraw Hill N.Y. (1957) DUGUE D.,and GIRAULT M., Analyse de variance et plans d'experience, Dunod, Paris, (1969) 337 (1982) BARATON M.I., and QUINTARD P., J. Mol. Structure,2 TURRELL G., Infrared and Raman Spectra of Crystals, Academic Press, London (1972)

424 17 18 19 20 21 22 23

2.25 (1977) QUINTARD P., Infrared Physics,g, 201 (1959) DWCKAERT G., Analyst.,E, LECOMTE J., Spectrom6trie dans l'infrarouge , Handbuch der Physik Licht und Materie II, Springer Vex-lag, Berlin (1949) PIRLOT G., Bull. Sot. Chim. Belg.',fi, 28 (1949) 298 (1947) SIEGFRIED W.D., and HASTING S.H., Anal. Chem.,l9, PAWLOWSKI L., VARDELLE M., and FAUCHAIS P., Thz Solid Films,%, 307 (1982) VARDELLE A., BARONNET J.M., VARDELLE M., and FAUCHAIS P., IEEE Plasma 417 (1980) Science ,P58 , GWONNET Jy FAUCHAIS P., Int. Round Table, Odeillo, Sept 1975 and private communications