Letter
The selective oxidation behaviour of WC–Co cemented carbides during the early oxidation
stage
Liyong Chen a ,Danqing Yi a ,?,Bin Wang a ,Huiqun Liu a ,Chunping Wu a ,Xiang Huang a ,Huihui Li a ,Yuehong Gao b
a School of Materials Science and Engineering,Central South University,Changsha 410083,People’s Republic of China b
Zhuzhou Cemented Carbide Cutting Tools Company Limited,Zhuzhou 412007,People’s Republic of China
a r t i c l e i n f o Article history:
Received 4February 2015Accepted 18February 2015Available online 2March 2015Keywords:
A.Ceramic matrix composite A.Cobalt
C.Selective oxidation B.EPMA B.AFM
a b s t r a c t
The oxidation experimental of WC–Co cemented carbides was carried out at 500°C in air.Selective oxida-tion phenomenon of Co phase in WC–Co cemented carbides was observed and proved by EPMA and AFM.At ?rst,the O content in Co phase increases quickly,and then tends to be stable after oxidation of 20min.In sharp contrast,no change of O content in WC phase was observed.Moreover,it was found that the oxide scales thickness of Co phase increased parabolically with oxidation time.And thermodynamic and kinetic analysis of oxidation was done.
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1.Introduction
Cemented carbides belong to a class of hard,wear-resistant,refractory materials in which hard carbide particles (WC,TiC,TaC,NbC,etc.)are bonded with a soft and ductile metal binder (Co,Ni,Fe,etc.).It is known to all that they are used in a wide range of applications,such as metal cutting,mining,construction,rock drilling,metal forming,structural components and wear parts.However,they are often exposed to various corrosive environ-ments,such as acid,alkali,salt,elevated temperature,which will result in the degradation of mechanical property of cemented car-bides products,such as hardness,strength and wear resistance,and ?nally will result in the decreasing of their service lifetime.For instance,the oxidation of WC–Co cemented carbides cutting tools at elevated temperature is one of the key factors in determining the lifetime of cemented carbides products.As a matter of fact,their service lifetime is greatly determined by their oxidation.The reasons are as follows:?rst of all,oxide ?lms with porous and cracks will form during the oxidation.These oxide ?lms are very easy to ?ake off,and will de?nitely decrease wear resistance [1–10].Additionally,it has been proved that the strength of the cemented carbides is weakened because of surface oxidation [4–6,11–13].Thus,the investigation of oxidation of cemented
carbides is signi?cant to the design and applications of these materials.
As for the oxidation of cemented carbides,many results have been reported [1–3,7–10,13–17].Kinetic analysis,the macro-,and micro-structure and the phase constitute of oxide scales are the most important aspects of the investigation of oxidation.Attaining the curves of mass gain or the thickness of the oxide scales vs.oxidation time by isothermal method and calculating the apparent activation energy (E a )by isothermal or non-isother-mal method are the major task of kinetic analysis.Barbatti et al.[7]have reported that the linear law or parabolic law of mass gain vs.oxidation time was in?uenced by temperature.E a was in?u-enced by many factors,such as consist of cemented carbides and oxidation temperature [2,3,8,9].The reported values of E a for WC–Co cemented carbides with the same chemical constituents are very different.For instance,the E a of WC–15Co (in wt%)was reported to be 190kJ/mol by Aristizabal et al.[9]and 234.1kJ/mol by Voitovich et al.[15],respectively.The great differ-ence of E a may be caused by instruments and/or experimental methods.
From the previous literature,we know that the oxide scales formed on a WC–Co substrate mainly consist of WO 3and CoWO 4,and a small amount of Co 2O 3and Co 3O 4[1–3,7–10,13].The WO 3phase has rather low resistance to oxidation because it contains numerous pores and cracks which may become a channel for the diffusion of O 2and CO https://www.doczj.com/doc/a27130975.html,paratively,the CoWO 4phase is denser than WO 3,and has better resistance to the oxidation.The
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?Corresponding author.Tel.:+8673188830263;fax:+8673188836320.
E-mail address:yiof?ce@https://www.doczj.com/doc/a27130975.html, (D.Yi).
previous researches have indicated that the oxidation resistance of cemented carbides was improved by the increase of Co content and the addition of cubic carbides,such as TiC and/or(Ta,Nb)C [1–3,7,9,14].However,the oxidation resistance was decreased when Co binder was partly substituted by Ni binder because of the decrease of complex oxide(Co,Ni)WO4content[15].
However,the above researches were mainly about the later oxidation stage of cemented carbides that the oxide scales had already covered the surface of the samples.The phenomena related to the early oxidation stage,such as selective oxidation and oxida-tion location(intra-grain,grain boundary or phase boundary),are not reported in any paper.And actually selective oxidation is a common phenomenon in alloys,such as iron-based alloy [18–20],cobalt-based alloy[21,22],steel[23–25],and so on.As a composite material,WC–Co cemented carbides consist of at least two phases,and have the potential for selective oxidation. Additionally,the results of thermodynamic calculation of the oxidation of WC and Co indicate that the oxidation of WC is easier than the oxidation of Co.However,in our exploratory experiment the contrary phenomena were found,and the preliminary results showed that Co was?rstly oxidised and WC did not.
Therefore,based on the results of exploratory experiments,a detailed study of the selective oxidation of WC–Co cemented car-bides during the early oxidation stage was carried out in this paper. Three approaches were used to prove that the oxidation of Co phase is prior to the oxidation of WC phase.The?rst approach is to reveal the changes of the oxygen distribution at the same loca-tion of a sample with the increasing oxidation time by element map analysis of electron probe X-ray micro-analyzer(EPMA).The second one is to measure the content change of W,C,Co and O, which is particularly important,of WC phase and Co phase with the increasing oxidation time by quantitative analysis of EPMA. And the third approach is to measure the thickness of oxide scales with the increasing oxidation time by atomic force microscope (AFM).
2.Materials and experimental
WC–15Co(in wt%)cemented carbides were provided by Zhuzhou Cemented Carbide Group Company Limited.The most of WC grains of sample are6–8l m.The size of specimens is 5mm?5mm?10mm.In order to obtain a smooth surface before oxidation,one of the5mm?10mm surface was ground and pol-ished.All specimens were wet ground using40l m and20l m dia-mond discs.After that,they would be polished using9l m and 1l m water-based diamond suspension.
Samples were oxidised in air at500°C between5and150min in a muf?e(±2°C).Fourteen samples were used in this paper.One of them was oxidised for different times.While the other thirteen samples were separately oxidised for0,5,10,15,...,55,60min.
The element mapping and quantitative composition analysis of samples were done by EPMA(JXA-8230)with tungsten?lament. The EPMA was operated under the following settings:accelerating voltage is15kV;probe current,dwell time and step size are about 11.3nA,70ms and93.8nm for element mapping,10.1nA,50ms and50nm for quantitative composition analysis.The thickness of oxide scales was measured by tapping mode AFM(multimode V)using silicon tip.Scan areas are20l m?20l m and 10l m?10l m and512lines per image were scanned with512 samples per line.
3.Results and discussion
Fig.1(a)shows the backscatter electron(BSE)image of WC–15Co sample,in which the dark areas represent Co phase contain-ing small amounts of dissolved W and C,and the grey areas represent WC phase containing a very small amount of dissolved Co.Element map analyses by EPMA were executed in the region displayed in Fig.1(a).In order to ensure the element map analyses by EPMA are at the similar same region,the sample was marked by Vickers indentation on the polished surface before oxidation.The average O contents of mapped regions vs.oxidation times are plot-ted in Fig.1(b).It should be noted that the results of O mapping are qualitative rather than quantitative.Before oxidation,the average O content is4%.High O content comes from chemical and physical adsorption of oxygen in air,and few oxide particles adhered to the polished surface.The average O content is10%,17%,20%,25%after oxidising for5,35,90,150min,respectively.The curve of average O content vs.oxidation time roughly follows a parabola.From0to 5min,the average O content increases rapidly;from5to35min, the growth rate becomes lower;after35min,the increasing of average O content becomes slow.Fig.1(c)–(f)shows the O distri-butions after oxidising for0,5,35,150min,respectively.From Fig.1(c)–(f),it is found that O content in Co phase goes up with the increasing oxidation time.In contrast,O content in WC phase has scarcely changed with the increasing oxidation time.
From Fig.1,it is seen that only Co phase was oxidised at500°C. In order to further con?rm the above result,samples were oxidised at500°C from0to60min and then quantitatively analysed by EPMA to ascertain the elements content.For each time interval,5 points of Co phase and5points of WC phase were measured respectively.
Fig.2shows the variation of the W,C,Co and O content in the Co phase and the WC phase with oxidation time.There is little change of C content in Co phase.And the W content in Co phase ?uctuates around9%without obvious change.However,Co and O content varies greatly with oxidation time.Within20min,Co content?rstly goes down quickly with the increasing oxidation time;over20min,Co content goes down slowly and tends to be constant.The change of O content in Co phase is contrary to the change of Co content.Firstly,it increases quickly and then tends to be stable.It is certain that Co phase was oxidised at500°C.In Fig.2(b),the W,C,Co and O content of WC phase all?uctuated around a constant value.O content is about zero.It is implied that WC phase was not oxidised at500°C.To sum up the above results, WC–Co cemented carbides have the selective oxidation character-istic at500°C.
The selective oxidation of Co phase in WC–Co cemented car-bides resulted in the change of surface topography characterised by the thickening of the oxide scales of Co phase.AFM is a very powerful technique for providing high-resolution three-dimen-sional(3-D)surface topography on the submicron scale has been used as an alternative or supplementary tool to other techniques for studying the selective oxidation behaviour of some materials during the early oxidation stage[26].In this research we also used AFM to measure the variation of surface topographies with oxida-tion time.Fig.3(a)shows the thickness variation of oxide scales of Co phase with oxidation time at500°C.Each data is an average of 20random measured values.Before oxidation,the height of WC phase is31±5nm higher than Co phase.However,after5min, the oxide scales thickness of Co phase increased to93±6nm. This suggested that the Co phase has been oxidised.And then, the oxide scales thickness of Co phase increased from141±9nm to215±14nm corresponding to oxidising for15min and 35min.After35min,the increasing of the oxide scales thickness of Co phase is slow.AFM topographies for0,5,15,35,60min oxidation are shown in Fig.3(b)–(f),respectively.The selective oxidation of Co phase is clearly seen from Fig.3.
The oxide scales thickness of pure Co in previous literature are 775nm(temperature500°C,time20min,P(O2)/P h=0.1)[27], 440nm(467°C,20min,P(O2)/P h=0.21)[28],360nm(505°C, 19min,P(O2)/P h=1)[29]and500nm(500°C,15min,P(O2)/
2L.Chen et al./Corrosion Science94(2015)1–5
P h=0.21)[30],https://www.doczj.com/doc/a27130975.html,pared the results in this paper with the previous ones,they are of the same order of magnitude. At a certain temperature,the oxidation rates of Co were found to remain the same(within experimental error)with the oxygen pressure varying from0.03MPa to0.33MPa[31].So,it is reason-able enough to compare our results with the previous results. However,it should not be neglected that the difference in experimental conditions,apparatus,and the method for measuring the thickness of oxide scales may result in the data difference reported by difference authors.
Chemical reactions for oxidising WC and Co are suggested as follows.
2=5WCesTtO2egT?2=5WO3esTt2=5CO2egTe1T2CoesTtO2egT?2CoOesTe2TThe values of standard Gibbs free energy(D G h)for reaction(1) and reaction(2)calculated by thermodynamic from the data in Thermochemical Data of Elements and Compounds[32]are à390kJ/mol andà355kJ/mol at500°C,respectively.The values of D G for reaction(1)and reaction(2)in air areà400kJ/mol and à345kJ/mol at500°C,respectively.Based on the above calculated results,it can be inferred that the oxidation of WC is prior to Co which is contrary to the experimental results.The reason for this contradiction may be attributed to the difference in oxidation kinetics of WC and Co.Unfortunately,all available data of oxida-tion rate and E a of WC and Co cannot be used for direct and accu-rate comparison because of the different experimental conditions (oxidation atmosphere,temperature,the characteristics of sam-ples,etc.)and data processing method.Ribeiro et al.[33]found that E a of the oxidation of WC powders(99%,<10l m)is between 120and220kJ/mol.And,Kurlov et al.[34,35]found that E a of the oxidation of WC powders changes from90to120kJ/mol with the average particle size from20nm to10l m.E a of the oxidation of Co is between75and160kJ/mol[28,29,31,36].E a of WC and Co overlap to some extent.Gulbransen and Andrew[27]found that the temperature of the oxidation of Co can be as low as200°C by recording changes in weight with vacuum microbalance
electron image of WC–15Co.(b)O content vs.oxidation time.(c–f)O mapping of Co and WC phases after oxidising at Fig.2.The content change of W,C,Co and O in Co phase(a)and WC phase(b)with the oxidation time.
technologies.In contrast,the initial oxidation temperature of WC measured by thermal analysis technologies is about500°C [34,35,37].So we can guess that the reason of selective oxidation of Co phase in WC–Co cemented carbides may be that the oxida-tion rate of Co is greatly higher than WC at500°C.
Based on the above results,the oxidation resistance of WC–Co cemented carbides is determined by Co binder which contains small amounts of W and C.The oxidation resistance of Co–W, Co–Cr and Co–Cr–W alloy is lower than pure Co in different degrees[38,39].In order to improve the oxidation resistance of WC–Co cemented carbides,changing the composition of Co binder by alloying of Co phase and optimisation of sintering process and/ or heat treatment might be effective in industry practice.In addi-tion,the selective oxidation of Co phase in WC–Co cemented car-bides bring about the change of wear mechanism for WC–Co cemented carbides tools in service.
4.Conclusions
This research has proved the selective oxidation behaviour of WC–Co cemented carbides at500°C in air by EPMA and AFM tech-nologies.In addition,the oxidation of Co phase is prior to the oxidation of WC phase.At?rst,the O content in Co phase increases quickly,and the O content tends to be stable after oxidation for 20min.In sharp contrast,there is no change of O content in WC phase because of no oxidation taking place.Besides,the oxide scales thickness of Co phase measured by AFM goes up parabolical-ly with the increase of oxidation time.In a word,the selective oxidation of Co phase in WC–Co cemented carbides is highly explicit which is proved by EPMA and AFM.This work inspires us that change of the component of Co binder is practicable to improve the oxidation resistance of WC–Co cemented carbides. Acknowledgements
The authors are grateful for?nancial support from the National Natural Science Foundation of China(General Program51474244) and Hunan Non-ferrous Metals Holding Group Co.,Ltd.(Project No.YSZN2013CL01).And we greatly appreciate the professor Yi group for their many valuable discussions.
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