Revista Latinoamericana de Metalurgia y Materiales, Vol. 20, N°2, 2000, 42-46

CHARACTERIZATION OF VANADIUM CARBIDES COATINGSPRODUCED BY THERMOCHEMICAL DIFUSIVE TREATMENT IN

MOLTEN SALTS: COMPOSITION AND RESIDUAL STRESSES.1 ,,1 , 2 2

V. Herrera ,Lo M. Fernandez .B. Aragon e lo Zamora o

1. Centro de Estudios Aplicados al Desarrollo Nuclear (CEADEN).Departamento de Análisis y Ensayos. Calle 30 #502 el 5ta y ra

, Miramar, Playa. C.Habana, Cuba.CP 6122; E-Mail: victoria@ceaden.edu.cu

2. Centro de Investigaciones Metalúrgicas. (CIME).Ave. 51 No. 23611 el 236 y 240, San Agustín,La Lisa. C. Habana, Cuba.

Resumen.

Las tensiones residuales están presentes en casi toda estructura ensamblada.La Difracción de Rayos X, entre otros usos, es una técnica no destructiva de medición de tensiones residuales en un

campo superficial del material.Los recubrimientos de carburos de metales de transición han ganado gran importancia en la fabricación de herramientas

gracias a su alta resistencia al desgaste.El tratamiento termoquímico combina la acción de la temperatura con condiciones de saturación por difusión de un

metal o aleación dado con metales o no metales, de manera que propicien la realización de reacciones químicas en una capasuperficial.

En el presente trabajo se efectúa la caracterización fásica de recubrimiento s de carburo de vanadio producidos sobreaceros de herramienta X12M y 09XBG (norma rusa) y se evalúa la tensión residual generada en el recubrimiento mediantela técnica de Difracción de rayos X. En la caracterización de las capas además se empleó la Microscopía Electrónica deBarrido.

Los recubrimientos obtenidos tienen espesores entre 6 y 12 um con una microestructura globular y se componen decarburo de vanadio de estructura NaCl. Los recubrimiento s presentan macrotensiones de compresión relativamente bajas,entre 0.3 y 0,5 GN. m-2. Se observa además ciertas microtensiones.

Palabras Clave: Recubrimientos de Carburo de Vanadio, Esfuerzos residuales, difracción de rayos X, acero paraherramientas, MEB.

Abstract.

Residual Stresses are present in almost a11assembled structures.X Rays Diffraction, 1S a versatile non - destructive technique, which finds a high valuable use for residual stressesmeasurement on the material surface.

Coatings of transition metals carbides have acquired a remarkable importance in the tool production, due to their highwear resistance.

The coupled action of temperature and diffusion controlled saturation of a given metal or alloy with other metals or non-metal s is the key element of thenno - chemical treatment. As result, surface chemical reactions are induced.

In this paper X Ray phase characterization of vanadium carbide coatings produced on tool steels X12M and 09XBG(Russian norm) is carried out. X Ray Diffraction Residual Stresses analysis is achieved on coating surface. Forcharacterization Scanning Electron Microscopy and Electron Probe microanalysis are also used.

Studied coatings are formed by vanadium carbide (VC) with a NaCl structure. Low compressive macrostresses (0.3 y0,5 GN. m-2.) are measured. Some microstresses are also observed. A globular microstructure is characteristic for thesecoatings.

Keywords: Vanadium Carbide Coatings; Residual Stresses; X-Ray Diffraction; Tool Steels; Sem

residual are presenr in almost allled srrucmre..dnal stresses are produced in technological

¡;:n:ceSS:5 su h as welding, heat treatment, galvanic anddiffusive coating, or as result of procedures

plastic deformation is involved. Generally,compressive stresses have been considered

cial for parts performance, due to the reduction ofice tensile stresses. Oppositely, tensile residual

~~::s can lead to unforeseen fractures.Therefore, the information on residual stresses

;;- nces to a large extent the concepts on technological¡;:nJcedure,s.

Coatings of transition metals carbide ha ve acquired aimportance in the manufacture of tools and parts

ID their high wear resistance.Therrno - chemical treatment causes surface chemical.ons where key elements are the coupled action ofrature and diffusion controlled saturation of a givenor alloy with metal s or non-metals. As result,sition, rnicrostructure and the stress level ofgs can be controlled.

As a way to raise the hardness, wear, cavitation andion resistance, the thermo- chemical treatment also

vides favorable residual stresses. Therefore, it findsrtant applications to increase the reliability andbility of parts.

The quality of diffusive coatings is characterized by:its structure and phase composition,its total or effective depth,the concentration of diffusing element;

• the fragile fracture capability under the action of alocalload;the homogeneity, continuity and uniformity of thedistribution of coating along the configuration of themetallic piece (configuration effect);

• the magnitude and gradients of residual stresses .Several coatings types are known, especially PVD

hysical Vapor Deposition) and the CVD (Chernicalapor Deposition) of titanium carbonitride and nitride

_-5], that have been those ones of more industrialímportance. Coatings of tungsten carbide, iron silicide,hrornium carbide have been also studied.

Several references on procedures to produce carbidelayers, with MC (M = W, V, Nb) type structures [6-10]are reported. They are applied on low alloyed steels toraise their wear resistance.

In this paper, phase characterization of vanadiumcarbide coatings onto tool steels X12M and 09XBG(Russian standard) was carried out. Residual stresses oncoatings are also measured.

Cylíndrical amples ofX12M and 09XBG steels withvanadium carbide coatings were obtained in molten saltmixtures with different sort and concentration ofreducing agent. Nominal chemical composition of citedsteels is shown on Table 1.

Depth profile for carbon, vanadium and iron in thecoating was measured by Electron Probe Microanalysisin a JEOL Scanning Electron Microscope.Phase composition was determined by X Ray Diffraction(XRD). Residual macrostress on produced coatings isalso evaluated by the sin2 t¡rmethod [12,13].XRD allows to measure the lattice strain E ~ '" directly inthe direction (\jf, <») of a rectangular coordinate systemxyz, where x- and y- axes líe on the sample surface planeand z axe is normal.

The angle \jf is formed between the normal to thesample surface Ns and the normal to diffractingcrystallographic planes N(hkl) in a plane normal to samplesurface.

The angle <p is forrned between the direction of themeasured stress cr~ on the sample surface and one of thex- or y-axes.Ns• N(hkl) and cr~ lie in a plane, normal to the samplesurface.

The strain € in the (rp, \jf) direction is expressed by theequation:

En = (dn-do)/do (1)

where, d ~ 'l' ". interplanar spacing for (hkl)crystallographic planes measured in the (rp ,\jf) directionon the stressed material.do- interplanar spacing for (hkl) planes measured in thenon stressed material.In an isotropic body the strain € ~ 'l' is given by theequation:

E ~ IJI = V2 S2 [ ( cr~ " cr33) sin2\jf + o 33 + (crl3 coso + cr23símp ) sin 2\jf] + SI [crn + cr22 + cr33] (2)

where, SI and Y2 S2 - X-ray elastic constants. They arefunctions of (hkl) planeso i i - normal components of stress ten sor i = 1,2,3cr ij - shear components.where,cr~=cr llCOS2 <p+ cr22sin2<p + 2cr12sin<p coso (3)

For comparison purposes, the theoretical powderdiffraction pattem of vanadium carbide was ca1culated.

44 Revista Latinoamericana de Metalurgia y Materiales, Vol. 20, N°2,2000

Table I. Nominal chemical composition of tool steels X12M and 09XBG [11]

STEEL C% Cr% Mo% Mn% Si% W%

X12M 1,45 - 1,70 11,0 - 12,5 0,5 - 0,8 - - -

09XBG 0,85 - 0,9 0,9 - 1,2 - 0,8 -1,0 0,15 - 0,35 1,2 - 1,6

3. ResuIts.

• Chemical composition.

The assessment of the chemical composinon ofcoatings is meaningful due to the influence of impuritieson coating perfection. Furthermore, chernical,mechanical, physical and tribological properties ofcoatings are conditioned by their phase stoichiometry.

In this study observed elements in coatings arecarbon, vanadium and iron. (fig.l and 2). They aredistributed homogeneously. However, in some cases acertain partial substitution of vanadium by iron wasobserved.

400

350

300

250

ene::s 200oo

150

100

50

OO 2 3

• Phase composition and microstructure.The rnicrostructure of coating controls in a great extentits mechanical and tribological propertiesCoatings thickness with values ranging from 6 to 12 umwas measured. These layers show a globular structurewith some pares (fig.2). Cracks at the interfacecoating/bulk metal are not observed. (fig.3).

X Ray Diffraction diagrams show a NaCI structurevanadium carbide VC (Table.II). In fig. 4 the calculateddiffraction pattem is given. Influence of rnicrostresseswas considered. Actually, experimental pattem exhibitsthis effect. The lattice parameter appears smaller thanthe calculated one. This could be related to differences inthe vanadium /carbon ratio.

9

Fig.l Depth profiles for carbon, vanadium and iron in a ve coating. SEM Electron Microprobe

•-,"~.~'r~I~ ~~

I

--w -- --••

6 7 84 5Depth,l'm

e _c~ - v~ -----=- FeKa-

v. Herrera y col./Revista Latinoamericana de Metalurgia y Materiales 45

Fig.2 VC Coating SEMFig 3b. Vanadium distribution in the coating. SEM Electron

Probe Microanalysis

• Residual Stresses.The residual stresses can be established on coatings,

due to deformations induced by growth defects, whichaffect locally the lattice pararneter; or by anisotropy ofthe elastic properties of grains during the growth of thelayer. [14].

In some coatings residual stresses can be present, thatare able to produce plastic deformation or evenrnicroscopic cracking.

In residual stress measurements vanadium carbide(333) peak (d = 0.0800 nm) was used. Line position fordifferent \jf orientations (O < sin2\jf <0.5) wasdeterrnined by a para bola profile fit around themaximum.

Fig.3a. Coating cross section.

Table II:Calculated and experimental XRD line intensities and positions for Vanadium Carbide VC (NaCl structure).

28teor. Iteor. Iexp. Dteor. Dexo. (hkl) F2P Iabs.

37.237 92.7 47.,7 2.4146 2.429 111 2836. 8 71.1343.267 100.0 100 2.0910 2.089 2 O O 5716 6 76.7762.849 55.8 31.8 1.4786 1.475 220 3748 12 42.8175.376 30.6 17 1.2609 1.254 3 1 1 1504 24 23.5279.366 17.3 5.7 1.2073 1.202 222 2796 8 13.3195.0lí 8.6 5.7 1.0455 1.041 4 O O 2249 6 6.61106.927 15.1 5.7 .9594 .956 331 969 24 11.62111.048 30.8 9.1 .9351 .930 420 1904 24 23.68129.119 36.4 11.4 .8537 .851 422 1672 24 27.95148.593 8.5 8.0 .8049 .800 333 731 8 6.52

46 Revista Latinoamericana de Metalurgia y Materiales, Vol. 20, N°2,2000

1SIl

(a) I - 27M I

io.., L • .1 ,11. ...l..

n'~ IIlIlo

Sil

Il31l 9IJ 11[]Sil 11l 13[] 1S[]

2 theta

~0r-----------------------------------'

150

(b) - ve (calculado) I

l:l§ 1008

O~----.-----.-----.-----.-----.---~:xl 50 70 110 1S0

2 thetaFig.4 Experimental (a) and calculated (b) diffraction pattem of

vanadium carbide coatings

The obtained strain distribution is cuasilinear (fig.5),pointing out the lack of shear components. It shows aweak texture effect, as indicated by the small oscillationaround the linearity. According to the slope of thisdependence, compression stresses are present.

149.0

-~

/

149.1 (333) line ve-.- experimental

linearfit

148.9

148.8

~ 148.7

148.6

148.5

148.4

148.3

0.0 0.1 0.2 0.3 0.4 0.5 0.6.2

SIIl\jf

. Fig.5 Strain distribution for vanadium carbide coating onto steeJ09XBG,

Reported values of Young Module E for titaniumnitride and carbide are between 200 and 600 GN. m ,2

[14]. The constant Y2 S 2 is re1ated with Poissoncoefficient E in the following way

According to [15] for vanadium carbide E =276GN.m -2 compression stresses between 0,70 ± 0,08 and0,49 ± 0,07 GN. m ,2 are calculated fram the measuredstrains. These stresses are lower to those ones reported in[14], where values from 2 to 3 GN. m'2 are obtained fortitanium nitride coatings (TiN) and values between 17and 20 GN. m,2 for titanium carbonitride (Ti(C,N).The observed halbwidth of diffraction lines gives ameasure of the microstresses in these systems. The emay be related to a high defect density or localcomposition gradients.

4. Conclusions.

Residual macrostresses yielded during thermo-chemical treatment in molten salts were measured forvanadium carbide coatings (NaCl structure) on tool steelX12M and 09XBG.Measured macrostresses arecompressive with values between 0.5 and 0,7 GN. m",The studied coatings show some microstresses.Produced coatings have a thickness between 6 and 12mm with a globular microstructure. Cracking at thecoatinglbulk metal interface was not observed.

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