investigation on multi-element ni–cr–mo–cu alloying layer by double glow plasma alloying...

Materials Chemistry and Physics 92 (2005) 340–347

Investigation on multi-element Ni–Cr–Mo–Cu alloying layer bydouble glow plasma alloying technique

Xu Jianga,∗, Xishan Xieb, Zhong Xuc, Wenjin Liua

a Laser Processing research Center, Mechanical Engineering Department, Tsinghua University, Beijing 10084, PR Chinab University of Science and Technology Beijing, Beijing, 100083, China

c Taiyuan University of Technology, TaiYuan, 030024, China

Received 2 April 2004; accepted 22 October 2004

Abstract

This paper describes an investigation of double glow surface alloying of low-carbon steel with Hastelloy C-2000 nickel-based surperalloy.Emphasis is placed on the effect of the source electrode voltage, cathode voltage, working pressure and parallel distance between sourceelectrode and cathode on the chemical composition and physical qualities of surface alloying layer. The results show that the total contentof alloy elements, thickness of alloying layer and absorbing alloy element rate have closely related with technological parameters. Thec ing layer.T ture. Thec icated thatt n steel.©

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ombination of SEM and XRD is used to investigate morphology and structure of the multi-element Ni–Cr–Mo–Cu surface alloyhe thermodynamic calculation was performed to predict the mole fraction of phase in the alloying layer as function of temperaalculated results is in agreement with the observation of microstructure of alloying layer. The corrosion experimental results indhe corrosion resistance of alloying layer formed on the stainless steel was super to that of alloying layer formed on the low-carbo

2005 Published by Elsevier B.V.

eywords:Double glow; Technological parameters; Ni–Cr–Mo–Cu multi-element alloying

. Introduction

It is well known that the reason of damage for most ofetallic components is relative to the surface properties ofaterials. Technique of surface treatments that result in spe-

ial properties have stimulated much interest for increasingardness, corrosion resistance etc. Professor Xu Zhong fromaiYuan university of technology invented a new techniqueor surface treatment, the double glow plasma surface alloy-ng technique, known as Xu-Tec/Xu-Loy process. Worldwideatents for Xu-Tec process have been granted in the Unitedtates, Canada, United Kingdom, Australia, Belgium, Francend Sweden[1]. The double glow surface alloying technique

s unique and hybrid plasma surface treatment techniquehich is the evolution of both plasma nitriding and sput-

ering techniques and develops in response to the need for

∗ Corresponding author. Tel.: +86 13311212937; fax: +86 10 62772353.E-mail address:[email protected] (X. Jiang).

higher quality alloy layer on the surface of cheap materThis technology employs low temperature plasma prodby a glow discharge to drive source materials atoms of omore element to be sputtered and then diffuse into thestrate’s surface. The depth of the alloying layer couldform several microns to 500�m, with alloying elements inconcentration of few percent to 90% or more. Mono-elemalloying of alloying-element of Ni, Cr, Mo, W, Ta, Al, Ti, et[2] and multi-element alloying of alloying elements of Ni–W–Mo, W–Mo–Cr–V, etc. have been studied[3–6]. Comparing to ion implantation or laser surface alloying, the douglow plasma technique is cheaper for many potential useorder to achieve a high quality surface alloying processglow discharge parameters must be selected carefully. Tare four parameters relating the glow discharge in the doglow process, i.e., source electrode voltage, cathode voworking pressure and distance between the source eleand cathode. This work introduces the effect of technoloparameter on the composition and thickness of multi-ele

254-0584/$ – see front matter © 2005 Published by Elsevier B.V.
oi:10.1016/j.matchemphys.2004.10.050

X. Jiang et al. / Materials Chemistry and Physics 92 (2005) 340–347 341

Ni–Cr–Mo–Cu surface alloying layer by double glow plasmaprocess.

2. Experimental method

The Xu-Tec process is performed in a vacuum cham-ber. Fig. 1 indicates the general principle of Xu-Tec pro-cess. There are three electrode:the anode and two negativelycharged members, the cathode (workplace) and the sourceelectrode. The source electrode is made up of the desired al-loying elements. With the two power supplies turned on bothcathode and source electrode are surrounded by glow dis-charge. One glow discharge heats the substrate to be alloyedwhile the second glow strikes the source electrode materialsfor supplying desired alloying elements. The desired alloy-ing elements travel toward the substrate and diffuse into thesubstrate materials surface forming alloying layer.

The surface alloying experiments were performed in Dou-ble Glow Plasma Surface Alloying Devise. Hastelloy C-2000(the composition is Ni 59, Mo 16, Cr 23, Cu 1.6, C <0.01 inweight percent) plate (130 mm× 50 mm× 4 mm) was usedas the source electrode for supplying alloying elements. Lowcarbon steel 1020 plate (80 mm× 25 mm× 3 mm) were usedas substrate. Orthogonal test is given inTable 1for choos-ing the process parameters such as source electrode voltage,c ourcee y fac-t lloye yingl reas-

TF

F

S 0W 0WD

ing mass of workpiece to the mass loss of source materials)are selected as evaluating target from the principle and de-mand of the double glow discharge plasma alloying process.The chemical compositions in the surface layer are analyzedby X-ray Energy Dispersive Spectroscopy (X-EDS).

Potentiodynamic anodic polarization curves were ob-tained at a sweep rate 20 mV min−1, starting form a mo-ment when the open circuit potential become steady afterimmersion of the specimen for about 10 min. A saturatedcalomel electrode and a platinum sheet were used as refer-ence and counter electrodes, respectively. An electrolyte usedwas 3.5% NaCl, 5% HCl and 5% H2SO4 solution open to airat 25◦C.

3. Results and discussion

3.1. Source voltage

The effect of source voltage on the content of alloy ele-ments on surface, thickness of alloying layer and absorbingalloy element rate are shown inFigs. 2–4. FromFigs. 2–4, thecontent of alloy element of surface of alloying layer, thicknessof alloying layer and absorbing alloy element rate increasewith increasing of source electrode voltage. Because of thee ge,t f ionbi 0 V,t A to7 den-s ormh lec-t entsi f the

F n thes

athode voltage, working pressure, distance between slectrode and cathode as experimental factor, and ever

or has four levels (treatment time 3 h). The content of alements of alloying layer on surface, thickness of allo

ayer and absorbing alloy element rate (the ratio the inc

Fig. 1. Schematic diagram of the Xu-Tec unit.

able 1actor and levels

actor Levels

1 2 3 4

ource electrodevoltage (V) 1050 1000 950 90orkpiece electrode voltage (V) 275 250 350 30orking pressure (Pa) 35 30 45 40istance (mm) 15 20 25 22.5

nergy of ion bombarding relating with sputtering voltahe higher source electrode voltage, the higher energy oombarding on the target is offered. As shown inFig. 5, when

ncreasing source electrode voltage from 900 V to 105he source electrode current increases linearly from 6.97.4 A. Since the increasing of a amount of output powerity and sputtering coefficient, which help especially to figh supplying desired alloy element from the source e

rode materials and gradient concentration of alloy elemn surface alloying layer. Correspondingly, the process o

ig. 2. Effect of source voltage on the content of alloy elements ourface of alloying layer.

342 X. Jiang et al. / Materials Chemistry and Physics 92 (2005) 340–347

Fig. 3. Effect of source voltage on the thickness of alloying layer.

diffusion of alloy elements toward substrate is accelerateddue to higher heating temperature.

3.2. Cathode (workpiece) voltage

The relationships of cathode voltage with the content ofalloy elements on surface, thickness of alloying layer andabsorbing alloy element rate are shown inFigs. 6–8. Theresults show that the content of alloy element, thickness ofalloying layer and absorbing alloy element rate increase withdecreasing of cathode voltage. Owing to cathode electrodeglow discharge, the substrate is heated to the process tem-perature; meanwhile, the surface of substrate is cleaned andactivated by ion bombarding, which conduce to absorptionand diffusion of alloy element in surface alloying layer. How-ever, there is a negative effect of increasing cathode voltage in

e.

Fig. 5. Effect of source voltage on the source electrode current.

double glow process, namely re-sputtering phenomenon andtherefore the absorption alloy element of alloying layer canre-sputter back to vacuum or even source electrode materi-als surface. For this reason, the cathode (workpiece) voltageshould be low, 250 V is appropriate in our research.

3.3. Working pressure

Figs. 9–11show the effect of working pressure on the con-tent of alloy elements on surface, thickness of alloying layerand absorbing alloy element rate. It can be seen that thereexists a peak of these evaluating target which occurs at theworking pressure of 35 Pa. The effects of working pressureon the glow discharge are complexity. On the one hand, whenthe working pressure is low, the intensity of ion bombarding

F n thes
Fig. 4. Effect of source voltage on the absorbing alloy element rat
ig. 6. Effect of cathode voltage on the content of alloy element ourface of alloying layer.


Fig. 7. Effect of cathode voltage on the thickness of alloying layer.

on the source electrode and activation of substrate are high,which are certainly favorable to diffusion of alloy elements ofsubstrate, but may give rise to inappropriate re-sputtering ofalloy elements. On the other hand, when the working pressureis high, despite the density of ion bombarding is increased,but the degree of back scattering phenomenon is increasedcorresponding, which is harmful to the traveling of sputteredparticles of desired alloy elements.

3.4. Distance of source electrode and cathode(workpiece)

Figs. 12–14show the relationships of the technologicaltargets with distance of source electrode and cathode. FromFig. 12, the content of alloy element on surface is independentof distance of source electrode and cathode; but with increas-

te.

Fig. 9. Effect of working pressure on the content of alloy elements on alloy-ing layer.

ing the distance of source electrode and cathode, the thicknessof alloying layer and absorbing alloy element rate decreased.The long distance make against the transport of particles ofthe desired alloy elements from the sputtered source materi-als and possibility of scattering loss of alloy element is in-creased. The short distance is propitious to form the thickalloying layer and high absorption of alloy elements. How-ever, the shorter distance isn’t fit to install sample and qualityof alloying layer isn’t also stable.

3.5. Chemical composition

The distribution of alloying elements in the composite sur-face alloying layer is shown inFig. 15. It is evident that con-tent of Ni, Cr, Mo, Cu in alloying layer decreases as the dis-tance from the surface increases. The gradient alloying layers

r.
Fig. 8. Effect of cathode voltage on the absorbing alloy element ra Fig. 10. Effect of working pressure on the thickness of alloying laye


Fig. 11. Effect of working pressure on the absorbing rate of alloy element.

are identically composed of alloying elements (Ni, Cr, Mo,Cu)-enriched layer, in which the composition is similar to thatof Hastelloy C-2000, and transition layer to the substrate.

3.6. Microstructure of surface alloyed layer andthermodynamic calculations

The microstructures of surface alloyed layer formed onlow-carbon steel by double glow plasma surface alloying isshown inFig. 16. It can be found that the interface of alloyinglayer and substrate characterize by metallurgical adhere andno porosity has been found. The surface alloying layer iscontinuous and compact. It can be clearly seen that whitephase distribute over the upper of alloying layer formed onthe 1020 low-carbon steel.

ent.

Fig. 13. Effect of distance between poles on the thickness of alloying layer.

Fig. 14. Effect of distance between poles on the absorbing rate of alloyelement.

Fig. 15. The alloy elements distribution of Ni–Cr–Mo–Cu alloyed layer.
Fig. 12. Effect of distance between poles on the content of alloy elem


Fig. 16. Microstructure of surface alloyed layer formed on low-carbon steel.

Fig. 17presents a typical X-ray diffraction (XRD) profileobtained from the alloying layer formed on the low-carbonsteel. It may be noted that carbide M6C and� phase emergehere in addition to the strong austenite (�) peak.

Owing to the gradient distribution of composition of alloy-ing coating, the phase structure of the alloying layer may bevaried as the distance from the surface increases. The ther-modynamic calculation can be a effective method to solvethis complicate problem.Fig. 18shows the results of ther-modynamic calculation by using the surface composite ofalloying layer formed 1020 low-carbon steel. All the pre-dicted equilibrium phases and their weight fractions at eachtemperature have been presented. The precipitated phases,such as carbide M6C and� phase, increase with decreasingthe temperature. At alloying temperature (1000◦C), the al-loying layer formed on low-carbon steel emerge precipitatedphase.Fig. 19shows the relationship of calculated phase (�

n vs. t

Fig. 17. XRD spectra of the surface alloyed layers formed on different sub-strates.

phase) fraction and layer depth (1020 low-carbon steel sub-strate). It must be noted that calculated precipitated phasefraction (� phase) is the largest at the surface of alloyinglayer and decrease with the increasing distance from the sur-face. The amount of� phase is relative to the content of Mo inthe alloying layer.Fig. 20shows the relationship of the con-tent of Mo in the alloying layer and thermodynamic drivingenergy of� phase. The thermodynamic driving energy of�phase increases with increase in the content of Mo in alloyinglayer, hence the precipitated� phase increases accordingly.The calculated results is in agreement with the observationof microstructure of alloying layer.

Fig. 18. Calculated phase fractio
emperature diagram for experimental.


Fig. 19. Calculated phase fraction and layer depth as function of the layer depth at 1000◦C (1020 low-carbon steel).

Fig. 20. The relationship of content of Mo in the alloying layer and thermodynamic driving energy of� phase.

3.7. Corrosion results

The potentiodynamic polarization tests of surface alloyedlayer on the low-carbon steel has been investigated in com-parison to 304 stainless steel in 3.5% NaCl, 5% HCl and 5%H2SO4 solution. The results are shown inTable 2. In a 3.5%NaCl solution, it can be clearly seen from theTable 1thatthe passive current densityip of the surface alloying layerformed on the low-carbon steel is lower than that of 304stainless steel; the pitting potentialEb of surface alloyinglayer formed on the low-carbon steel is the same amount asthat of 304 stainless steel. In a 5% HCl solution, the passivecurrent densityip of the surface alloying layer formed on thelow-carbon steel is about half of that of 304 stainless steel; thepitting potentialEb of surface alloying layer formed on thelow-carbon steel is 145.91 V higher than that of 304 stainlesssteel. In a 5% H2SO4 solution, the passive current densityip

Table 2Electrochemical properties of alloying layer formed on low-carbon steel in3.5% NaCl, 5% HCl and 5% H2SO4 solution

Specimen Pittingpotential(mV)

Passivecurrent density(�A cm−2)

3.5% NaCl solutionAlloyed layer on low-carbon steel 190 31.622304 stainless steel 190 0.6767

5% HCl solutionAlloyed layer on low-carbon steel 45.91 316.228304 stainless steel −100 562.341

5% H2SO4 solutionAlloyed layer on low-carbon steel 930 7.294304 stainless steel 870 51.286


Fig. 21. Corrosion micrograph of surface alloyed layer formed on the low-carbon steel (a) and 304 stainless steel (b).

Table 3Corrosion results of 200 h immersion tests in 20% HCl solution and 20% H2SO4 solution

Specimen Corrosion rate in 20% H2SO4 solution (g m−2h−1) Corrosion rate in 20% HCl solution (g m−2h−1)

Alloying layer formed on low-carbon steel 0.1930 0.2359304 stainless steel 3.8400 4.2700

of the surface alloying layer formed on the low-carbon steelis one order magnitude lower than that of 304 stainless steel;the pitting potentialEb of surface alloying layer formed onthe low-carbon steel is 60V higher than that of 304 stainlesssteel. The corrosion morphology of alloying layer formed twokinds of steel after the potentiodynamic polarization tests in a3.5% NaCl solution are shown inFig. 21. The surface of 304stainless steel is rougher than the surface of alloying layer.

The results of 200 h immersion corrosion tests of alloyinglayer in 20% HCl solution and 20% H2SO4 solution (Table 3)indicate that the corrosion rate of alloying layer formed onlow-carbon steel is one order magnitude lower than that of304 stainless steel.

4. Conclusions

(1) The optimum processing parameters of theNi–Cr–Mo–Cu alloying layer by double glow plasmaalloying are: source electrode voltage, 1050 V; substratevoltage, 250 V; working pressure, 35 Pa; and paralleldistance between the source electrode and the substrate,15 mm (treatment time is 3 h).

(2) The thermodynamic calculation was performed to pre-dict the mole fraction of phase in the alloying layer as a

function of temperature. The calculated result is in agree-ment with the observation of microstructure of alloyinglayer.

(3) The corrosion resistance of the new Ni–Cr–Mo–Cu al-loying layer is superior to that of 304 stainless steel,which proves that Xu-Tec process is feasible, high ef-ficient and non-polluting surface modification method incorrosion resistance of materials.

References

[1] Z. Xu, et al., Method and apparatus for introducing normallysolid materials into substrate surfaces, U.S. Patent No. 4,520,268(1985).

[2] Z. Xu, Z. Wang, F. Gu, Double layer ion metallic cementation, Trans.Met. Heat Treat. 1 (1982) 71 (in Chinese).

[3] Z. Xu, R. Liu, Z. Xu, Surface alloying simplified, Adv. Mater. Pro-cesses 12 (1997) 33.

[4] B. Fan, Z. Xu, Plasma Ni–Cr surface metallizing for A3 steel sheet,Heat Treat. Met. 9 (1988) 42 (in Chinese).

[5] Z.H. Li, X.P. Liu, J.X. Zhao, et al., Surf. Coat. Technol. 131 (2000)579–581.

[6] X. Zhang, X.S. Xie, Z.M. Yang, J.X. Dong, Y. Gao, Z. Xu, T.H.Zhang, A study of nickel-based corrosion resisting alloy layer ob-tained by double glow plasma surface alloying technique, Surf. Coat.Technol. 131 (2000) 378.

investigation on multi-element ni–cr–mo–cu alloying layer by double glow plasma alloying...

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