TITANIUM CARBIDE COATINGS ON STEEL : STUDY OF THE CONDITIONS OF ELABORATION AND OF SUBSTRATE-COATING INTERACTIONS

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TITANIUM CARBIDE COATINGS ON STEEL : STUDY OF THE CONDITIONS OF ELABORATION

AND OF SUBSTRATE-COATING INTERACTIONS

A. Derre, F. Teyssandier, M. Ducarroir

To cite this version:

A. Derre, F. Teyssandier, M. Ducarroir. TITANIUM CARBIDE COATINGS ON STEEL : STUDY OF THE CONDITIONS OF ELABORATION AND OF SUBSTRATE-COATING INTERACTIONS.

Journal de Physique Colloques, 1989, 50 (C5), pp.C5-445-C5-453. �10.1051/jphyscol:1989556�. �jpa-

00229586�

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JOURNAL DE PHYSIQUE

Colloque C5, suppl6ment au n05, Tome 50, mai 1989

TITANIUM CARBIDE COATINGS ON STEEL : STUDY O F THE C O N D I T I O N S O F ELABORATION AND O F SUBSTRATE-COATING INTERACTIONS

A. DERRE, F. TEYSSANDIER' and M. DUCARROIR'

E.T.C.A., 16 bis avenue Prieur de la CGte d'Or, F-94114 Arceuil Cedex, France

'CNRS-IMP, UniversitB, avenue de Villeneuve, F-66025 Perpignan Cedex, France

Resume

La tn6thodologie des plans d'exMrience a et6 appliqde au &p6t de Tic sur des aciers hypoeutectoi'des faiblement allies. Des d l e s descn'ptiis de I'epaisseur du revQtement et de la variation de poids ont et6 obtenus et cor6kSs a des calculs thermodynamiques dans le domaine exp6rimental envisage. Les surfaces de kponse correspondantes ont ete utilisees pour etudier quelques proprietes. I1 a ete observe que pour une epaisseur constante, le coefficient de frottement du couple Ticlacier varie fortement avec la morphologie qui est fonction du mecanisme de dep6t.

Les valeurs elevees des microduretes sont attribuees a un effet de taille de grains. Les contraintes r6siduelles ont ete evaluees. I1 a ete montr6 que la decarburation du substrat dam les conditions geMralement utilis6es ne peut gtre completement evitee.

Abstract

The methodology of experimental designs was applied to the deposition of Tic on weakly albyed hypoeutectoYd steels. Models describing the coating thickness and weight change were obtained and correlated with thermodynamic calculations. The corresponding response surfaces were used to study some properties of the deposits. The friction coefficient of a titanium carbidelsteel couple has been observed to present a particularly large variation with respect to the surface morphology, which depends on the experimental conditions.

The highvalues of mimhardness wich mere measured mere presumed to be induced by a grain size effect. The residual stresses bere evaluated. It has also been Shown that substrate decarburization cannot be completely avoided with the usual coating conditions used by previous authors.

Titanium carbide has been used as wear protecting coatings for many years. Initially deposited on cemented carbide cutting tools (inserts), its application field extended with the diversity of the materials used as substrate. In particular, as an exemple a large range of steels are processed industrially by chemical vapor deposition, in order to give them useful surface properties. Their main composition fields are presented on the isothermal section of the iron-carbonchromium phase diagram in fiiute 1. This is of course a s i m p i i i diagram, since other minor alloying elements (V. W, Ni..) are also present "A" corresponds to highly alloyed high carbon steels (too! steels) [I]. In the "B "field, the less alloyed hypereutectoi'd steels correspond to the cold working steels, used to manufacture parts such as pumhes, extruding nozzles or ceramic powders compactiqn die. Low alloyed hypereutedcii'd steels (the "C "field) are used for bearings[2].

The high albyed hypoeutedoid steels of the "D" field are used to protect parts against chemical conosbn or high temperature damage (valves, push-rods, cams,...). On the other hand, the low alloyed hypoeutectoi'd steels ("E " ) a r e nowadays only rarely coated with titanium carbide. This kind of steel is widely used in mechanical engineering, and such a p m s s i n g would increase the reliability of parts working under severe conditions.

The purpose of this wrk is thus to study the coating conditions of titanium carbide on low carbon sbeei and to determine the influence of various parameters on the properties of the coated substrates. This work also deals with solidlgas reactions and diffusion phenomena occuring during coating.

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989556

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2- EXPERIMENTAL DEVICE

The coatings ate achieved on inductively heated polished samples (diameter = 16mm, thickness = 5mm) in a cold wall reactor. Temperature is measured by means of a thermocouple welded by spark discharge at the bottom of a hole drilled into the steel. The flow rates of the permanent gases (hydrogen and methane) are set by means of mass flowmeters. The molar fraction of the titanium tetrachloride is cont~lled by a dew point evaporator [3]. At the end of the experiment, the sample is cooled under a hydrogen flow.

3- EXPERIMENTAL RESULTS AND DISCUSSION

3-1 Determination of the response surface

Most of the experimental situations require the examination of the effect5 of varying tvvo or more factors. It has been shown [4 -51 that in order to obtain a complete exploration of such a situation it is not sufficient to vary one factor at a time, but all combinations must be examined in o&rto elucidate the effect of each factor and the possible ways in which one may be modified by the variation of the others. In this case a generally useful technique is provided by factorial design. When the expeded number of experiments generated by the choice of the factors is too high a fractional factorial design may be used.

This method has been described elsewhere [6] and applied to the present study with the following observed responses :

-

^(Am): weight variation of the sample

.

-

(e) : thickness of the coating measured by a spherical wear technique.

Among all the factors, temperature (T), total pressure (P), the initial molar fraction of methane in the gas phase (X,J and duration of the deposit were previously determined to be preponderant parameters [6]

The response surfaces were then obtained according to an experimental design, proposed by HOKE IT].

The various levels selected for each of the four parameters are presented in table 1 ( Xc = 0 means that all the carbon comes from the substrate). The resutting response surface can be described by a second degree polynomial expression.

e =7.99+2.45 T + 1 . 8 t + 1 . 0 7 ~ ~ + 0 . 8 5 ~ + 1 . 1 7 ~ ~ + 0 . 7 6 ~ ~ ~ + 1 . 1 4 ~ ~

-

1.35~: +2.13$

The relative importance of the factors agrees wII with the results of the previous experimental design.

Some isoresponse surfaces are plotted against T,Xc,t for several total pressures (figure 2 and figure 3).

The surfaces representing thicknesses are almost flat because of the weak interactions between the T, t, atxi Xc parameters. As they are parallel to the (111) plane the variation of one of them may be counterbalanced by a variation of the opposite amount of one of the two others in order to obtain the same thickness. Similar behavior is observed for mass variation under atmospheric pressure.

The shape of the surfaces under reduced pressure is no longer a plane as a consequence of a strong coupling effect between two main parameters XcP.

Due to the difficulties encountered in explaining these results a composite response (R) has been calculated (ratio of the mass variation, Am, to thickness, e) :

R = M e

Its variations are plotted versus T and t in figure 4.

If mass variation is only growth rate dependent , this ratio would be constant whatever the coating conditions and would be equal to 2.5 according to the geometry of the sample and the titanium carbide density.

~ h , -

, ,

,k K?,u,, -I IH I-

;,

c;;nfi;;md

iixle;

atmospheric pressure (figure 4a,

X,

= 0.1). Under reduced pressure, the ratio (R) is lower and can even be negative (grey part of the surface), which can be interpreted as the simultaneous growth (experimentally observed) of a titanium carbide coating and a global weight loss of the

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sample. This unexpected behavior is attribued to the formation of iron chloride FeCI2, due to a chemical exchange reaction between titanium chlorides and steel. As a matter of fact thermodynamic equilibrium calculations show the increased formation of FeCI2 when pressure is decreasing. In an open system, this formation of iron chloride is enhanced and leads to a global weight loss of the substrate-coating couple.

Titanium carbide as coating is mostly used for its tribological properties. Preliminary resutts have been used to characterize the coatings in term of friction. Three series of coatings with a constant thickness (5pm) were prepared under different conditions according to the previous response surface; their behaviors are presented in figure 2a (series A, 6 and C). As scratch tests showed that the corrosion of the substrate was responsible for poor adhesion, the total p ~ s s u r e was fixed at 1 atm in order to reduce the formation of iron dichloride.

In the pin on disc type friction tests,

the

pin is composed of a 35 NCD 16 (9335) steel treated at 1800 MPa.

The samples "A", made without methane in the gas phase, present a friction coefficient which, after trans*Non, leads to a final friction coefficient of 0.25-0.3 (figure 5). The "B "series behave diffe~ntly, since the final friction coefficient is 0.650.7 (figure 6).

This variation can be related to the surface morphology of the samples. The samples "A" are rather smooth (Ra = 0.2 pm) (figure 5b), presenting a "paving block" structure. These blocks correspond to the austenite grains at the beginning of the experiment. This particular relief results from the d i i i o n a l growth of the titanium carbide. On the other hand, samples prepared under high methane molar fraction

O(,

= 0.1) differ by the presence of an extra, much thinner relief (figure 6b). The fact that the "paving block" type of relief remains seems to prove that the diffusional component of growth is still present under these conditions, at least at the very beginning of deposition. The thinner relief can be assigned to the, growth mode using methane from the gas phase. On the other hand, this last relief is responsible for a high friction coefficient.

The third series of samples, "C", obtained for an intermediate molar fraction of methane, presents both types of friictional behaviour (A and B) according to the samples. No intermediate friction coefficient (between 0.3 and 0.7) is observed.

This non-reproducibility can be interpreted as a competition between the

two

growth modes. This is confirmed by the surface morphology (figure 7). In thiscase the relief associated with the growth mode wiEn using methane appears first on the thickest parts of the coating (in the middle of the "paving blocks"). So it seems that the chosen coating conditions correspond to the boundary beyond which the growth mode from the gas phase is preponderant. The presence or absence of the extra relief related to the preponderant growth mode extensively modifies the friction coefficient. The non-reproducibility could thus be due to an uncertainty in controlling the experimental parameters.

3-3 Microhardness promrties

A high surface hardness is one of the most attractive feature when coating metallic parts with ceramics such as titanium carbide. In figure 8, the V i e r s hardnesses, are plotted against the indentation load for b w coatings prepared under different conditions (T = 1050"C, P = 1 atm, t = 90mm,

X,

⁼0 and 0.1). The measured thicknesses are respectively 9 and 15 pm.

In order to measure only the properties of the deposit, the volume of the plastic zone developed under the indenter must not exceed the thickness of the layer. As a rule of thumb these requirements are met when the indentation depth (h) is less than a tenth of the coating thickness (e). This empirical rule has been confirmed in the case of titanium carbide [8] and the corresponding validity range is presented in figure 7. The microhardness of both coatings are similar.

The measured values (3500kg/mm-~-100~) seem rather high in comparison with most of the published va!ues; On the

-

other hand, the

harriness

of coatings at higher temperature (T = 1600'C) [3] decreases to 2200 kg mm-' under a load of 100 g. This wide range of variation may be related to the modification of the grain

size

from 100 pm at lEOO°C to 50 to IOOnm at 1050°C as demonstmted by SEM (figm 9) 2nd TEM

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C5-448 JOURNAL DE PHYSIQUE

@gum 10). This change of microstNchrre b mainly mponsible for both the hardening of the material by grain size effect and the scattered measurements found in the literature.

3-4 Residual stresses

In addition to

the

high hardness and the low friction coefficient of the deposit it is also necessary to ensure a good adhesion to the substrate. Large amounts of residual macro stresses stored in the coating can be responsible for it flaking off. For these reasons sVesses have been measured by the X- ray d i c t i o n method [91. The measurement of the lattice spaang dhkl with respect to the Jr angle bebeen the perpendicular to the plane and the perpendicular to the surface, allows the calculation of the residual stress in the coating (equation 1) assuming a plane stress state. Figure 1 1 shows the variation of

the

diffraction angle 20 versus

+

"

³⁶⁰

^q,

^sin2^I(equation 1)

2

, , e -

2

e+,

=

-

---

- -

cotg

e+,,

E n

With E = 455 GPa [lo] and v = 0.19 (Poisson's ratio) respectively, a compression stress of 2000 MPa is obtained. A calculation based on the theory of elasticity and assuming a perfect adhesion at the interface [ I 11 -6 1 allows us to conclude that the mismatch beween the thermal expansion ooeff'ints of steel ( a =12 10 K- ) and titanium carbide (a = 7.4

lo4

K-l) is mainly responsible for

the

measured stresses .When the thiness exceeds 10 pm the stress value d m p rapidly to 780 MPa. A partial relaxation of

stms

is observed when

the

elastic energy stored in the film, and which increases with its thickness, is high enough to allow ^crack propagation within titanium carbide. This cracking starts at the interface, as shown in the figure 12, and induces considerable flaking off

.

3-5 Modifications of the substrate composition

The main problem encountered when coating steel pieces with titanium carbide is the decarburization of the subsrate [12-1Sj. This phenomenom has been mentioned in the paragraph 3-2. In

the

case of low alloyed

hypoeutectoid steels, the change in composition of the substrate is harmful since it provokes a severe drop in the mechanical properties of the steel.

The direct measurement of decarburization by microprobe analysis leads to large discrepancies. This decartxlrization has thus been qualitatively observed by Vickers indentations (bad 100 g). All the samples were oil quenched at the end of the experiment in order to prevent further modification of the carbon distribution by a nucleation and growth process. Figure 13 shows the resuk measured on three samples 35 CD4 steel (41 35) coat& with a methane molar fraction of 0,0.05 and 0.1 respectively (P = 1 atm, T = 1050"C, t = BOmn, XTiC14 = 8 1 iT3). For

X ,

⁼0, the drop of hardrsss fmm the middle of tksutxtfate (x = 2.5mm) to tts deposit interface (x = 0 mm) is considerable and the decarburization affects almost all the substrate thickness (total thickness 5 mm). While an increase in the methane molar fraction (Xc ⁼0.05) reduces the

level and depth of decarburization, it does not completely prevent this phemmenom, even for the highest concentration (0.1). Afurther increase in methane concentration would give homogeneous nudeation into the reactor under atmospheric pressure.

As a consequence, substrate decarburization cannot be completely avoided within the studied experimental field which corresponds to the conditions generally used.

The main factors controlling the chemical vapor deposition of titanium carbide on steel have been determined by means of an experimental design. The role of the total pressure on steel corrosion has been interpreted using thermodynamic equilibrium calculations. This corrosion is induced by the iron- titanium chemical exchange by means of chlorides, and is enhanced under reduced pressure. Coatings of identical thickness have been prepared according to the response surfaces previously determined. Their friction coefficients are nevertheless strongly affected by the coating conditions, mainly by the methane molar fraction. This variation can be explained as the

result

of a competition between Wo growth modes : carbon

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coming from methane or from steel

.

Diffusion of carbon from the steel substrate cannot be completely avoided under normal coating conditions, even by increasing the methane molar fraction until homogeneous nucleation appears. The mismatch between the thermal expansion coefficients of steel and TIC induces compressive stresses in the coating. This stress can be high enough to allow crack propagation within titanium carbide and subsequent scaling.

This study has been completed recently and solutions allowing the prevention of steel decarburization have been found [20]. These results will be presented in a further publication.

REEFERENCES

[l] T. SADAHIRO, S. YAMAYA, K. SHIBUKI, N. UJllE

Planseseminar, Reutte Tirol, Bettray 32 Vorabdnrck, (1977), Vol. 2,32 , 1-13 [2] H.J. BOVING, H.E. HINTERMANN, G.STEHLE

Journal of the American Society of Lubrication Engineers, Septembre, (1981), 534-537 [3] F. TEYSSANDIER

TMse, Gtenoble, 31 Octobre (1 986)

[4] R. PHAN TAN LUU, D. FENEUILLE, D. MATHIEU

"Methodologie de la recherche ex@rimentale", Cycle d'actualisation des connaissances IPSOI, Universitt5, rue Henri Poincarr5,13397 Marseilles Cedex

[5] ^G.E. P. BOX, L. R. CONNOR, W.R. COUSIN, 0. L. DAVIES, F. R. HIMSWORTH, G. P.

SlLLlTrO

"The design and analysis of industrial experiments", 0. L. Davies editor. (1963)

[q

A. DERRE, M. DUCARROIR, F. TEYSSANDIER

J. Electrochem. Soc., Vol. 136, No. 3, March (1989), 853-858

m

A.T. HOKE

Technometrics, Vol. 17, No. 3, (1975), 375 [8] E. HUMMER, A.J. PERRY

Thin Solid Films, 101, (1 983), 243-251 [sl G. MAEDER, J.L. LEBRUN, J.M. SPRAUEL

Materiaux et Techniques, Avril-Mai, (1981), 1351 49

[lo]

G.V. SAMSONOV, J.M. BlNlTSKll

Refractory Compounds Handbook, Metallurgy, Moscow, (1976) [I 11 A. DERRE

"DeNt chimique en phase gazeuse de carbure de titane sur acier", TGse, Universite de Perpignan, 24 Novembre (1 988)

[12] W. RUPPERT

GLASKI F.A. (Ed), Proceedings of the Ill International Conference on CVD, American Nuclear Soc., Hinsdale, (19721, Salt Lake City USA, 34G351 + 754

[13] P.J.M. VAN DER STRATEN, G. VERSPUI Philips tech. Rev. 40, (1982)' No.7.204-210 [14] A.J. PERRY, E. HORVATH

Metals and Materials, October, (1978), 37-40 [15] K. ROSER

WAHL G. , BLOCHER J.M. ,VUILLARD G.E. (Eds), Proceedings of the Vlll International Conference on CVD, Electrochem. Soc., Princeton, (1981), Paris FRANCE, 586-597 [16] P.P.J. RAMAEKERS, F.J.J. VAN LOO

BLOEM J. , VERSPUI G. , WOLFF L.R. (Eds), P w e d i n g s of the IV European Conference on CVD, (1983), Eindhoven THE NETHERLANDS, 546-552

[lTj S.G. YOON, H.G. KIM, J.S. CHUN

Journal of Materials Science 22, (1 987), 2629-2634 [18] A.J. PERRY, E. HQRVATH

Thin Solid Films, 62, (1979), 133-143

[lq P.J.M. VAN DER STRATEN, M.M. MICHORIUS, G. VERSPUI

BLOEM J. , VERSPUI G. , WOLFF L.R. (Eds), P m e d i n g s of the IV European Conference on CVD, (1983), Eindhoven THE NETHERLANDS, 553-567

1201 A. DERRE, F. TEYSSANDIER

Brevet No F 88 10553,04 Aoi3 (1 988)

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Fiaure 1: isothermal section of the iron-carbon-chromium phase diagram at 1000°C

Table 1: variation range of the experimental parameters

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(b)

Fiaute 2: thickness i-sponse (5 pm) with respect to T, t and Xc; (a) P = 1 atm; (b) P = 5 atm Temperature T ("C)

Duration of the deposition t (mn) Fraction of methane Xc

Total pressure P (atm)

LEVEL

-

1 950

30 0 0.005

0 1000

60 0.05 0.07

+ 1 1050

90 0.1

1

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30 t (mn> 90

30 t <mn> 90

(a) (b)

Fiaure 3: wight isoresponse (5 mg) with respect to T, t and Xc; (a) P = 1 atm; (b) P = 5

1 f l

atm

30 t <mn) ⁹⁰ t <mn) 90

(a)

(b)

Fioum 4: variation of the wight/thickness ratio with respect to T and t for Xc = 0.1 ; (a) P = 1 atm;

(b) P = 5

lom3

atm

Fiaure 5: tribological study of samples "A" made without methane

in

the gas phase; (a) friction coeffiient with respect to the time;

(b)

surface morphology

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C5-452 JOURNAL DE PHYSIQUE

Fiaure 6: tribobgical study of samples "B" achieved under high methane molar fraction; (a) friction coefficient with respect to the time; (b) surface morphology

6500 6000 5500 5000 4500 4000 3500 3000 2500 2000 1500

Eg!n%E surface morphology

of samples "C" obtained for an ¹⁰⁰⁰₂₀ ₃₀ ₄₀ ₅₀ ₆₀ ₇₀ 80 90 100

intermediate molar fraction of

methane Figure 8: Vickers hardness in respect with the indentation load for two coatings achieved with (Xc = 0.1) or without methane in the gas phase (Xc = 0)

Figure 9: microstructure of coating Fag% 10: K~cE,+~WE of coating obtained at

16000~2

(SEM) obtained at 10WC (TEM)

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2 SIN Y

153 Î Î Î Î Î

0,O 0,1 0 2 0 3 0 4 0,s 0,6

Fiaure 11 : variation of the diffraction angle 28 versus sir?Y

Fiaure 12: crack propagation within titanium carbide coating

Fiaure 13: Vickers hardness of the steel substrate versus the distance from the titanium carbidelsteel interface