NITRIDE AND CARBIDE COATINGS SYNTHESIS ON THE SURFACE OF REFRACTORY METALS BY LASER ACTION

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NITRIDE AND CARBIDE COATINGS SYNTHESIS ON THE SURFACE OF REFRACTORY METALS BY

LASER ACTION

A. Uglov, M. Ignat’Ev, A. Gnedovets, I.Yu. Smurov

To cite this version:

A. Uglov, M. Ignat’Ev, A. Gnedovets, I.Yu. Smurov. NITRIDE AND CARBIDE COATINGS SYNTHESIS ON THE SURFACE OF REFRACTORY METALS BY LASER ACTION. Journal de Physique Colloques, 1989, 50 (C5), pp.C5-727-C5-733. �10.1051/jphyscol:1989586�. �jpa-00229619�

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JOURNAL D E PHYSIQUE

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

NITRIDE AND CARBIDE COATINGS SYNTHESIS ON THE SURFACE OF REFRACTORY METALS BY LASER ACTION

A.A. UGLOV, M.B. I G N A T I E V , A.G. GNEDOVETS and I . W . SMUROV B.A. Baikov Institute o f Metallurgy of t h e U.S.S.R. Academy o f Sciences, Leninsky pr. 49, Moscow 117334, U.S.S.R.

R6sumB

-

Les r e s u l t a t s expdrirnentaux de l a synthdse laser-plasma de com- p

- d

e metaux r e f r a c t a i r e s ( n i t r u r e s , carbuses, oxycarbures) s u r l a surfaces de s o l i d e s sont presentes. Les procedes physico-chimiques qui ont l i e n sont l ' e f f e c t de l ' i r r a d i a t i o n l a s e r des materiaux, en presence d'une atmosphere gazeuse, sont discutds.

Abstract

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Experimental r e s u l t s on t h e laser-plasma synthesis of c o w pounds of r e f r a c t o r y metals on the surface of s o l i d s ( n i t r i d e s , carbides, o q c a r b i d e s , etc.) are presented. Physico-chemical processes taking place during the e f f e c t of l a s e r r a d i a t i o n on t h e materials in the gas atmosphere i n a wide range of pressures a r e discussed.

The use of highly concentrated energy f l u x e s (HCEF) f o r the production of coatings with s p e u i a l p r o p e r t i e s (high microhardness, corrosion r e s i s t a n c e , etc.) on components is a highly promising method. The ITCEFs u s u q l y i n lude the f l u x e s i n which the density of the energy f l u x Q exeeds 10 W/cm

5

( e l e c t r o n and ion beams, l a s e r r a d i a t i o n focused on the small-diameter area, f l u x e s and bunches of low-temperature plasma, e t c

.

⁾

.

The high density of the f l u x makes i t possible t o obtain within a s h o r t period of time (several ms) high temperatures on the surface of s o l i d s ; these temperatures a r e higher than the melting point and cannot be produce'd by l e s s concentrated energy fluxes. A plasma cloud starts t o form i n the v i c i n i t y of the s o l i d with increasing f l u x density. A s a r e s u l t of recent extensive research i n t o the plasma and laser-plasma processes, the HCEFs, e s p e c i a l l y l a s e r r a d i a t i o n (LB), can now be used t o produce a r e a s on the surface of s o l i d s and, i n many cases, coatings with s p e c i a l physico-chemi- c a l p r o p e r t i e s /'I, 2/.

The physico-chemical processes taking place during the e f f e c t of

LR

on the materials i n the gas atmosphere i n a wide range of pressures axe usually associated with the development, i n the v i c i n i t y of the surf ace of t h e s o l i d , of a low-threshold o p t i c a l breakdown leading t o the formation of a plasma bunch whose evolution determines the development of phenomena on the surface

O f s o l ids.

The A.A. Baikov I n s t i t u t e of Metallurgy of the USSR Academy of Sciences i s carrying out intensive research i n t o the physical ^andphysico-chemical processes taking place during the combined e f f e c t of plasma generated by the e f f e c t of LR which has passed through the plasma bunch. The combination of

Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1989586

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C5-728 JOURNAL DE PHYSIQUE

t h e processes occuring i n the conditions of LR i s r e f e r r e d t o a s laser-plasma treatment /2/.

The i n v e s t i g a t i o n s i n t o the physical and physico-chemical processes taking place i n the course of the laser-plasma e f f e c t on metals may be divided i n t o

t h r e e main directions.

1. Experimental and t h e o r e t i c a l examination of the o p t i c a l breakdown i n dense gases under the e f f e c t of LB in the v i c i n i t y of materials with various p r o p e r t i e s ; examination of the evolution and physical p r o p e r t i e s of the plasma cloud.

2 . Examination of the changes i n the physico-chemical p r o p e r t i e s of the sur-

f a c e l a y e r s of m a t e r i a l s a f t e r the laser-plasma e f f e c t , including the main geometrical, phase, and s t r u c t u r a l c h a r a c t e r i s t i c s of the zones formed a f t e r the e f f e c t of LR and plasma.

3. Formulation of t h e o r e t i c a l considerations on the mechanism of surface synthesis of various inorganic compounds and the processes of oxidation and reduction of metals.

The first d i r e c t i o n associated with the examination of the opticaJ. breakdown Of dense gases i n LB has been a c t i v e l y followed in our research. Qn impor- t a n t r o l e i n these i n v e s t i g a t i o n s i s played both by e ~ e r i n e n t a l investiga- t i o n s and mathematical modelling (simulation) of the process /3/.

I n recent years, the second and t h i r d d i r e c t i o n s have been fallowed in the A.A. Baikov I n s t i t u t e of Metallurgy /4,5/ and a number of other organisati- ons /6-8/. The i n t e r e s t ercpressed i n . these i n v e s t i g a t i o n s is associated with the f a c t t h a t the detected phenomena can be u t i l i s e d i n t h e development of metallurgical and processing processes. A number of processes may be used f o r hardening the surface l a y e r s of materials and s e v e r a l other processes appear s u i t a b l e f o r the formation of surface l a y e r s and coatings with new p r o p e r t i e s .

The experiments described i n t h i s a r t i c l e were c a r r i e d out i n a number of l a s e r systems with various energy and space-time c h a r a c t e r i s t i c s of radia- t i o n (GOS-30M, GOS-300, GOS-1001, Kvant-16, UPN-102, eta.). The experimental setups were s i m i l a r and d i f f e r e d only i n s d l d e t a i l s . X e t a l l i c specimens were placed i n a high-pressure chamber which w a s f i l l e d with a gas with con-

t r o l l e d composition and pressure; LR was introduced i n t o the chamber through a s p e c i a l window. The LR parameters (pulse time, energy f l u x density, etc.) were inspected during t h e experiments, and the development of the processes

i n the zone of i n t e r a c t i o n of LR with metals was recorded by high-speed f i l - ming. The duration of the r a d i a t i o n p u l s e s equalled u n i t s of ms, and conti- nuous

LR

was used i n a number of e ~ e r i m e n t s . The plasma-forming gases were C02 (pressure 1-55 atm), Hz, A r , He, N2, CH+ (pressure 1-140 atm) and, i n some cases, mixtures of these gases. I n a d d ~ t i o n 60 the e f f e c t of LR on ma- t e r i a l s i n a dense gas atmosphere, a t t e n t i o n was a l s o given t o the e f f e c t of pulsed and continuous LR on the materials immersed i n transparent carbon- bearing l i q u i d s .

The coatings and zones of a c t i o n of LR were examined by metallograpby

,

X-ray d i f f r a c t i o n , o p t i c a l , e l e c t r o n microscopy s and other methods of analysis.

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LASER-PLBSDdB SYNTHESIS O F NITRIDES OF EESUCTORY METALS

The formation of n i t r i d e zones and coatinga on the surface of metals i s of g r e a t importance f o r a number of components since the n i t r i d e s are characte- r i z e d by high wear r e s i s t a n c e , high chemical s t a b i l i t y i n various corrosive media, high r e s i s t a w e t o f i r e , and number of other p r o p e r t i e s which a r e important f o r a p p l i c a t i o n i n p r a c t i c e .

The promising nature of laser-plasma production of n i t r i d e s i s determined by the foUowing advantages i n c o m p ~ i s o n with the t r a d i t i o n a l methods:

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-

the p o s s i b i l i t y of s t r i c t l o c a l i z a t i o n of n i t r i d e d zones ( t h i s i s g r e a t importance f o r a number of applications);

-

high r a t e s of synthesis of the n i t r i d e zones;

-

the p o s s i b i l i t y of producing n i t r i d e coatings with the p r o p e r t i e s d i f f e - r i n g from those of the i d e n t i c a l coatings produced by other methods.

We s h a l l examine the s p e c i a l f e a t u r e s of the synthesis of surface l a y e r s of the n i t r i d e s of r e f r a c t o r y metals using a Nd l a s e r with pulse time 1 ms.

The laser-plasma synthesis of titanium n i t r i d e a t high nitrogen pressures w a s described f o r the first time i n /9/ where the important r o l e played by

the surface l a s e r plasma i n the formation of the n i t r i d e l a y e r was reported.

Subsequent i n v e s t i g a t i o n s were c a r r i e d out t o deternine the r e l a t i o n s h i p between the dynamics of l a s e r plasma; the conditions of action of LR and t h e formation of a s p e c i f i c structureof n i t r i d e l a y e r s f o r a wide range of r e f r a c t o r y metals, such as T i , Z r , H f , Ta, N b , Mo, etc. X-ray d i f f r a c t i o n a n a l y s i s showed t h a t i n i r r a d i a t i o n of the metallic s h e e t s in nitrogen a t a pressure of p = 40 atm and higher, n i t r i d e s whose composition is s i m i l a r t o stoichiometric form on the surface in the case of T i , Z r , H f

,

whereas i n t h e case of Nb, Ta, Mo these n i t r i d e s a r e similar t o lower n i t r i d e s of the Me$ type. bfetallographic examination revealed differences i n the s t r u c t u r e s of the zones of a c t i o n caused by the parameters of t h e process.

We shall examine these differences u s i w T i a s an errample

/ l o / .

A t nitrogen pressures near the atmospheric pressure, the n i t r i d e d zone. c o g s i s t s of a t h i n l a y e r of titanium n i t r i d e ⁽

'

monolayer

*

⁾w i t h a columnar-dendritic s t r u - c t u r e and of a narrow band of the ^& -phase o r a mixture of the ^CL + ^& and

&

+

TiPJ phases whose tbtal thickness v a r i e s i n the range 1-5 mcm. These lay-

e r s a r e followed by a s o f t e r l a y e r of the s o l i d s o l u t i o n of nitrogen i n alpha metal and by t r a n s i t i o n l a y e r whose microhardness is lower than t h a t of the parent metal.

The increase of nitrogen pressure causes changes i n the s t r u c t u r e of the zone of synthesized n i t r i d e . The thickness of the n i t r i d e d l a g e r and of the l a y e r of the & -phase increases t o 450-200 mcm, whereas the thickness of the l a y e r of the s o l i d s o l u t i o n i n alpha titanium decreases u n t i l t h i s l a y e r completely disappears a t a nitrogen pressure of around 40-60 atm (Fig.1).

t

-

7

-

I I I I

0 X ) W ) 6 0 80 100

PRESSURE, atm

Big. 1

-

Dependence of t% thickness of ti anium n i t r i d e l a y e r on nitrogen pressure. ¹- q = 6 . 5 * 1 0 w/c&, 2 -4.10'. j - 1 . 5 - l o 6 .

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JOURNAL DE PHYSIQUE

200 400 600 800

TIME, mcscc

F i g , 2

-

Dependence of the o p t i c a l d e n s i t y of n i t r o g e n plasma on time.

r

=

1 m s , q = 3.106w/cm2, p n 7 4 a t m .

A t nitrogen p r e s s u r e s of p > 40 a t m , the microstructure of t h e zone of a c t i o n contains s e v e r a l 'monolayers' of n i t r i d e which a r e placed on top of each o t h e r and separated by bands of the E -phase. It i s evident t h a t these changes i n the s t r u c t u r e of the synthesized l a y e r s O f the n i t r i d e should be a s s o c i a t e d w i t h the s e l f - o s c i l l a t o r y processes taking place in the ' l a s e r beam-plasma-target s u r f a c e t system a t p r e s s u r e s exceeding approximately I 0 atm /91/. These changes may be explained as follows. The v a r i a t i o n s of t h e o p t i c a l d e n s i t y of plasma with time a r e shown i n Fig. 2 which g i v e s t h e time dependence of the c o e f f i c i e n t of r e l a t i v e transmittance of IIE from the t a r g e t i n the experiments with the nitrogen plasma a t various d e n s i t i e s of the IIR /12/.

The e f f e c t of HCms on metals i s accompanied by the formation of micro- and macroscale s t r u c t u r e s i n the melt and gas phase. The beam r a d i u s of HCEF r0 i s the n a t u r a l s c a l e f o r t h i s d i v i s i o n . The i n e q u a l i t y 1 << ro a p p l i e s t o t h e c h a r a c t e r i s t i c dimension of the microstructure 1

,

whereas f o r the large-scale s t r u c t u r e it is i t s c h a r a c t e r i s t i c dimension h of the order of beam r a d i u s h-ro

.

The dimension of the s t r u c t u r e r e f l e c t s the s c a l e of the most e f f e c t i v e i n t e r a c t i o n of t h e s p e c i f i c s t r u c t u r e of the melt and the gas medium with HCEF. The large-scale s t r u c t u r e s a r e determined by the p u l s a t i o n s of the temperature of the melt and the d e n s i t y of the gas medium formed as a r e s u l t of excessive s e l f - o s c i l l a t i o n s o r owing t o the use of HCEF p e r i o d i c i n r e s p e c t of time, or due t o the combined e f f e c t of both f a c t o r s .

I n c e r t a i n conditions t y p i c a l of the given specimen and process parameters, t h e r e a r e disruptiona i n the d i s t r i b u t i o n of n i t r i d e l a y e r s caused by t h e development of turbulence and movement of t h e melt i n the pool (Fig. 3).

The appearence of turbulence may be caused by t h e build-up of the self-os- c i l l a t i o n s i n the system.

The microhardness of the l a y e r s a l s o changes i n accordance w i t h tihe v a r i a - t i o n of the s t r u c t u r e of the zone of a c t i o n of LR i n the depth; the most

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Fig. 3

-

Devel ment of turbulence i n a T i melt (nitrogen, p = 70 atm, q = 3.10~ W/C.? z = l ms).

marked changes a r e detected i n the t r a n s i t i o n from the n i t r i d e l a y e r t o the l a y e r of the of the s o l i d s o l u t i o n and from the l a t t e r

ts

the p a r e n t metal.

The t y p i c a l values of microhardness a r e equal t o , kg;f/mm : f o r the n i t r i d e l a y e r 4000

-

2000, f o r the s o l i d s o l u t i o n l a y e r 300

-

900, f o r the t r a n s i - t i o n t o the parent metal 200. The microhardness within the limits of each l a y e r depends on t h e process parameters.

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EFF'ECT OF CONTIMJOUS LASER RADIAllION OW BIIMIAIS IN NITROGEN A T M O S m We shall discuss b r i e f l y the processes of synthesis of n i t r i d e compounds during the e f f e a t of continuous l a s e r r a d i a t i o n with the wavelength of A = 1.06 mom i n the nitrogen atmoephere on metals i n the pressure range I

-

⁸⁰

atm, I n lmer-plasma treatment of T i and Z r s h e e t s , the thickness of the n i t r i d e l a y e r was equal t o 1% mom a t the atmospheric pressure and dropped t o 20 =om with the pressure increasing t o 80 atm. The c r o s s s e c t i o n s of ti- tanium n i t r i d e f o r the pressures ?, 30 and 80 atm are shown i n Fig. 4.

Fig. 4

-

Cross s e c t i o n s of the %ones of e f f e c t of continuous IiR on T i .

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C5-732 JOURNAL DE PHYSIQUE

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LASEEL-PLASMA S Y N I I H B I S OF SUBPACE LAYERS OF CARBIDES OF REFRAGTORY The experimental set-up used i n s y n t h e s i s of carbides on the surface of components of r e f r a c t o r y metals i s i d e n t i c a l w i t h t h a t used a l s o i n the s y n t h e s i s of n i t r i d e s , only carbon-bearing gases C02 and CH4 a r e used a s the plasma-forming gases.

We s h a l l d i s c u s s t h e main s p e c i a l f e a t u r e s of laser-plasma s y n t h e s i s of the c a r b i d e s /13/. The thermochemical e f f e c t of the plasma cloud formed i n the v i c i n i t y ~f the surface of metals i n the atmosphere of chemically a c t i v e carbon-bearing gases makes it p o s s i b l e t o synthesize on t h e i r surface the carbide, oqycarbide o r carbohy&i.de coatings, depending on t h e process parameters and the gas used.

I n GO2

,

the carbide s y n t h e s i s processes were examined a t t h e pressure va- rying from 5 t o 60 atm. The d e n s i t y of the r a d i a t i o n f l u x of a pulsed Nd l a s e r ( 2 -1 m s ) w a s 'lo6

-

107 w/cm2. The i n i t i a l m a t e r i a l s were the metals of t h e groups IV-VI with an impurity content not higher than 0 , s .

The dependence of the thickness of synthesized ,compound on the gas pressure c o n t a i n s a n extremum and a t a pressure of p = 30 atrn reaches i t s maximum value of approximately 90-100 mcm. With a f u t h e r increase of t h e GO2 pres- s u r e the thickness of the carbide l a y e r decreases. For C02 pressure of p = 58 atm, the thickness of the carbide l a y e r i n the c e n t r e of the zone of ac- t i o n of WR does not exeed 5 mcm, whereas a t t h e edges i t s thickness increa- s e s t o %ens of micrometers. A t a gas pressure of 25-35 atrn the examined zone has a oomplioated s t r u c t u r e c o n s i s t i n g of a l t e r n a t i n g u s i a l l y t h r e e l i g h t and t h r e e dark layers. The microhardness i n the t r a n s i t i o n rom the

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l a y e r t o l a y e r smoothly v a r i e s from 1600-1800 t o 1100-1200 kgf/

.

^The

formation of a n u l t i l a y e r s t r u c h r e of t h e c a r b i d e s i s evident;Jy l i n k e d w i t h f l u c t u a t i o n s of the o p t i c a l d e n s i t y of the plasma and, consequently, w i t h t h e o s c i l l a t i o n s of the surface temperature; t h i s was a l s o d e t e c t e d

i n carbide s y n t h e s i s , i.e., i n the development of s e l f - o s c i l l a t @ r y proces- s e s i n the LEi-body surface system.

X-ray phase a n a l y s i s of the zones shows t h a t oxycarbides of t h e metal form on t h e surface of t e e metal i n the GO atmosphere. For example, a t a f l u x d e n s i t y of q

=

5.10 w/cm2 ( t h e targe% was made of T i ) and a C02 pressure of p ^zs 30 a t m the titagium oxycarbide T i c ~ 0 0 . p

,

and o w c a r b i d e ZrGo.~Oo.q7 f orma on Z r i n the same condi?~ons. ~ t h an increase of the g a s p r e s s u r e , t h e composition of the oqycarbides i s displaced t o higher carbon content and lower oxygen content. S p e c i f i c a l l y , a t a C02 p r e s s u r e of p

=

50 atm a pure carbide without ozygen forms.

PRESSURE, atm

B i g . 5

-

^Diameter (a) and depth ( b ) of the zone of carbidizing i n the e f f e c t of LR on Z r i n r e l a t i o n t o methane pressure.

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I n the methane atmosphere, the syn%hesis of metal carbides under t h e e f f e c t of LR is characterized by the following. The diameter of the zone i n which zirconium carbohydride is synthesized assumes t h e minimm value a t a methane pressure of p ^xSO aiim (Fig.5), and t h e thickness of the carbide l a y e r r a p i d l y drops. This i s caused by the e f f e c t of competing processes a f f e c t - i n g the formation of t h e plasma bunch, i o n composition, and its i n t e r a c t i o n w i t h the metal surface.

We s h a l l d i s c u s s b r i e f l y the laser-plasma s y n t h e s i s of carbides i n l i q u i d s . The i n v e s t i g a t i o n s o a r r i e d out i n the accordance w i t h the above experimental set-up were conducted i n v a r i o u s carbon-bearing l i q u i d s , such a s pen- t a n e , hexane, hectane, t o l u o l . A s i n methane, carbohydrides of the group TV metals form on the surface of t h e s e metals, whereas carbohydrides or carbides can form on the s u r f a c e s of t h e metals of the groups V and V I , depending on the treatment conditions. For example, i n l a s e r treatment of metal- l i c t a r g e t s i n t o l u o l , the tungsten carbide forms on the surface of tungs- t e n ( t h e composition is similar t o s t o i c h i o m e t r i c ) , whereas a carbide w i t h composition NozC forms on t h e s u r f a c e of a Eo sheet. D i f f r a c t i o n p a t t e r n s of t h e zones of s y n t h e s i s on the s u r f a c e of Ta s h e e t show simultaneously l i n e s of the tantalum carbohy&i.de and Ta2C carbide. The r e l a t i v e hydrogen content of the carbohydride i n the r e s u l t a n t c o a t i n g s depends on s e v e r a l f a c t o r s , such a s the Oensity of the 322 f l u x , t h e chemical composition of carbon-bearing l i q u i d , the thickness of the l a y e r of t h i s l i q u i d on the surface of the metal s h e e t , e t c .

6

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CONCWSIONS

The experimental d a t a described i n t h i s a r t i o l e r e p r e s e n t p a r t of t h e inve- s t i g a t i o n s embracing a wide range of g a s p r e s s u r e s and compositions, parameters of the specimens, and a l s o t h e space-time and energy parameters of IiB. The r e s u l t s i l l u s t r a t e c e r t a i n p o s s i b i l i t i e s of laser-plasma treatment.

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! 6

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/lo/

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