Article ID:1003-6326(2000)03-0405-03
TiC-Fe coatings prepared by flame spray synthesis process①
LIU Chang-song(刘长松),H UANG Ji-hua(黄继华),
ZHAO Yong(赵　勇),LIU M u(柳　牧),YIN Sheng(殷　声)
School of Materials Science and Engineering,
U niversity of Science and Technology Beijing,Beijing100083,P.R.China
A bstract:A new process,flame spray synthesis(FSS),has been developed fo r producing ceramic-containing composite
coatings.By combining self-propagation high-temperature synthesis(SHS)and flame spraying,the cermet-based material w as synthesized and deposited simultaneously.TiC-F e coa tings were deposited from commercial ferrotitanium,iron and
g raphite pow ders by the flame spraying synthesis process.M icrostructure analyses revealed tha t T iC was sy nthesized dur-
ing spraying,and that submicro n and round TiC particles w ere dispersed within an iron matrix.F lame-spray sy nthesized coatings were co mposed of alternate soft and hard lay ers,w hose hardness were3.0～6.0G Pa and11～13G Pa,respec-tively.
Key words:T i-C;coating s;flame spray;SHS Document code:A
1　INTRODUC TION
It is generally accepted that wear resistance is a consequence of a specific favorable combination of hardness and toughness.Because of their brittle na-ture,pure hard ceramic materials cannot be used fo r all protective coating s.Fine-grained multiphase coat-ing s composed of hard materials dispersed in a metal matrix w ould be tougher and more resistant to crack propagation than pure ceramic coatings w hile ensuring w ear resistance im parted by the hard ceramic phase. M ultilayer coating s in w hich interfaces are parallel to the substrate interface can also limit crack propag a-tion,thus increasing the toughness[1].
M ultiphase coatings com posed of mixed hard phases and metals have been currently produced by plasma spraying mixtures of metal and ceramic pow-ders,metal-coated ceramic pow ders and agg lomerated metal and ceramic powders[2].In m any cases,how-ever,these plasm a-sprayed multiphase
coating s are not com posed of lamellae containing finely dispersed hard phases.They can contain angular particles of a size larger than the lamella thickness.
In o rder to obtain a homogeneous microstructure and to ensure good wettability of the ceramic particles by the metal matrix,thus improving the bonding be-tween the metal matrix and the ceramic particles,a so-called plasma spray synthesis,or reactive plasma spraying process has been developed for producing ce-ramic-containing composite coatings[3～5].However, plasma spraying units are too ex pensive for the Chi-nese thermal spraying factories w here the flame spray technique is widely used[6].
It is noted that SHS is an in-situ technique fo r preparing materials because of the follow ing advan-tages:hig h purity of products,low energy require-ments,sho rt reactio n time and relative sim plicity of process.When the reaction between reagents is initi-ated,it will maintain without any other energ y[7].
Co nsidering the national conditio ns of China, TiC-based coatings were prepared by a new process, called Flame Spray Sy nthesis(FSS)combining SHS and flame spraying in this study.The TiC-Fe com-posites in the process were produced by the exother-mic reaction betw een ferrotitanium,iron and g raphite.The microstructure and characteristics of the coatings are described below.
2　EXPERIMENTAL
Com mercial ferrotitanium,iron and graphite pow ders w ere used as raw materials for preparation of micropellets and spray pow ders.Table1gives the chemical analyses of these pow ders.X-ray diffraction analysis show ed that ferro titanium mostly consists of Ti and FeTi(Fig.1).
The raw materials ferro titanium and iron pow-ders were separately milled in alcohol and then mixed together.The batch compositions were prepared by adding g raphite and binder.The mix ture contains a slig ht excess of carbon to com pensate the loss of Table1　Chemical composition of commercial
raw materials(%)
M aterial
Compos ition/%
Ti Si Al S P C Fe Ferrotitanium65.121.50.510.0220.0250.15Balance Iron>99
Graphite99.5
Vol.10　№.3        Trans.Nonferrous Met.Soc.C hina        Jun.2000①Received date:Jun.2,1999;accepted date:Oct.21,1999
Fig .1　X -ray diffraction patterns of
raw materials and coatings
carbon during spraying .For this purpose ,the calcu -lated carbon titanium atomic ratio co rresponds to 1.2.The batch w as adjusted to obtain 60%(volume fraction )TiC phases in the spray sy nthesized coat -ing s .After the solid micropellets w ere agg lomerated ,they w ere then sieved to -150～+300mesh .The resultant pow ders w ere flame sprayed on 10cm ×10cm ×0.8cm mild steel substrates w ith the process pa -rameters listed in Table 2.
Table 2　Flame spraying parameters
O xygen pressure /M Pa A cetylene pressure /M Pa
Protective gas
pressure /M Pa
Spray
distance /mm
0.8
0.09
0.5
150
  Scanning Electron Microscopy (SEM )w as car -ried out on polished samples .X -ray diffraction with
copper K αradiatio n w as used to characterize the reac -tion products and coatings .Vickers microhardness measurements were conducted under a 0.5N load .3　RESULTS AND DISCUSSION
The fundamental reaction in the flame spray sy n -thesis of TiC -Fe coating s takes place exothermically between titanium and graphite .When iron is added into the mix ture ,it w ill promote the reaction be -tween elements by dissolving them in an iron bath [8].Thus ,a tem perature as low as 1200℃,co rrespond -ing to the melting point of ferrotitanium ,is obtained to initiate the dissolution of carbon [5,8].It is worth mentioning that the starting reaction temperature is lower than by using pure elements ,namely titanium and g raphite ,w hose reaction is initiated at approxi -mately 1600℃[9]
.
This is w hy ferrotitanium w as used to take the place of titanium .In this case ,iron plays a role no t only as a diluent but also as a reac -
tant .
After injected into the flame ,the reactive micro -pellets (com prising ferrotitanium ,graphite and iron )
fo rm TiC and iron acco rding to the following reac -tion :
FeTi +Ti +2C ※2TiC +Fe (1)Fig .1gives the X -ray diffraction patterns of starting materials and coatings .
Although the calculated carbon titanium atomic ratio corresponds to 1.2in the reactive micropellets ,the coatings mostly consists of TiC and Fe ,and C is difficult to be identified .However ,when the sy nthe -sis was conducted in an electric arc furnace w here rapid heating and cooling could be obtained in an at -tempt to simulate conditions of spraying ,an excess of
carbon within the matrix w ould be found [10]
.The loss up to 20%(mole fraction )of graphite here could be the oxygenated carbon during spraying process .
Fig .2show s the back scattered SEM image of TiC -Fe coating s fabricated by flame spray synthesis process .It is found that these coating s are composed of alternate ,laminated dark ,g ray and w hite layers .X -ray analy sis show s that the coating s mostly consists of TiC and Fe .The dark areas have microhardness values in the range of 11～13GPa .Therefore ,the dark layers would be TiC -rich ,containing very fine and rounded TiC crystals dispersed in an iron -based matrix ,as show n in Fig .3.As a matter of fact ,the compositional analysis performed by energy dispersive spectroscopy w ith X -
ray microanalysis revealed that they are titanium rich w hile the w hite areas seem to be rich in iron .The iron -rich layers should also con -tain TiC but to a lesser extent .Indeed ,the pseudobi -nary TiC -Fe phase diagram predicts that iron can con -tain 3.8%TiC at the eutectic temperature .As a re -sult ,those iron -rich layers have a hardness of 3.0～6.0GPa .The Ti content of those g ray areas w ill be varied with the shades of color ,so their hardness varies between 7.0and 10GPa .
The presence of iron -rich layers w ithin coatings could result from inadequate micropellet fabrication or their partial breaking -up when they enter the flame .
Fig .2　Back scattered SEM image of flame
spray synthesized TiC -Fe coating s
·
406·Trans .Nonferrous Met .Soc .China          Jun .2000
Fig.3　Back scattered SEM image of
TiC-rich lamellae w ithin flame spray
sy nthesized TiC-Fe coatings
Iron-rich micropellets o r fragments of them are re-sponsible most likely for the fo rmation of iron-rich layers in coating s.
It is noted that TiC g rains w ithin the TiC-rich layers are very fine(<1μm)and round.When TiC is combustion sy nthesized from pure Ti and C mixture w ith a molar ratio of1.0and without any metal addi-tion,it is composed of agg lomerated angular grains w ith a size about15μm.This can be ex plained as fol-lows.With the addition of iron,the combustion tem-perature becomes lower and the g rain size becomes sm aller since the g rain grow th of TiC is an exponen-tial function of the combustion temperature;also, w hen the layers are deposited on the substrate,the rapid cooling prevent the g row th of TiC grains.
The peculiar microstructure of the com posite coatings,containing very fine and round TiC com-posed of alternate TiC-rich and TiC-poor layers,is expected to play a major role in their tribological properties[11].Fine-grained carbides can achieve an increase in yield strength through dispersion and g rain size mechanisms,and modify the plasticity of the ex-posed surface.What's more,alternate hard and soft layers would increase the toughness by limiting crack propagation.4　C ONCLUSIONS
1)A new process,flame spray synthesis(FSS), has been developed for preparing coatings containing hard ceramic phases w ithout a need to reach the melt-ing point of these ceramics.
2)TiC-Fe coatings were deposited by the flame spray sy nthesis process.The flame-spray-sy nthesized
coatings w ere composed of alternate soft and hard lay-ers,whose hardness is3.0～6.0GPa and11～13 GPa,respectively.
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(Edited by PENG C hao-qun)
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Vol.10　№.3      　TiC-Fe coatings prepared by flame spray synthesis process

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