Surface & Coatings Technology 205 (2011) 4627–4631

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Surface & Coatings Technology

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Ti-coated SiC particle reinforced sintered Fe–Cu–Sn alloy

Y.H. Wang a, X.H. Zhang a,b, J.B. Zang a,⁎, E.B. Ge a, J.H. Zhang a, X.Z. Cheng a

a State Key Laboratory of Metastable Materials Science and Technology, College of Materials Science and Engineering, Yanshan University, Qinhuangdao 066004, P.R. Chinab Hebei Vocational & Technical College of Building Materials, Qinhuangdao, Hebei 066004, PR China

⁎ Corresponding author. Tel./fax: +86 335 8387679.E-mail address: diamondzjb@163.com (J.B. Zang).

0257-8972/$ – see front matter © 2011 Elsevier B.V. Aldoi:10.1016/j.surfcoat.2011.04.015

a b s t r a c t

a r t i c l e i n f o

Article history:Received 29 January 2011Accepted in revised form 1 April 2011Available online 9 April 2011

Keywords:Fe–Cu–Sn alloySiCTi coatingMechanical properties

Ti-coated SiC particles were developed to improve the wear resistance of Fe–Cu–Sn alloy metal matricesdesigned for diamond tools. The phase structure of the Ti-coated SiC particles was investigated by X-raydiffraction. Ti coating on SiC was composed of Ti5Si3, TiC, and Ti. Excellent interfacial bonding between SiC andthe matrix was realized. The SiC/iron alloy composites, prepared by hot pressing at 820 °C, were studied bywear and bending strength tests, and scanning electron microscope. For the composites reinforced byuncoated SiC particles, the wear resistance was improved, but the bending strength decreased. Thecomposites with Ti-coated SiC particles outperformed the composites with uncoated SiC particles in bothwear resistance and bending strength tests.

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© 2011 Elsevier B.V. All rights reserved.

1. Introduction

Metal-bonded diamond tools have superiormechanical and thermalproperties, and are widely used inmaterials processing [1-3]. Themetalbonding materials are usually alloys composed of elements such as Co,Cu, Sn, W, and Ni [4-7]. For example, Co-based alloy is an excellentmetal bond for diamond tools because of its higher red hardness, goodchemical compatibility and satisfactory wear resistance at the proces-sing operations [8-10]. Co, however, is a strategic metal and is thuscostly. Furthermore, due to the high toxicity of Co, exposure to metalpowders or dust during the cutting processes is particularly hazardous[10,11]. For these reasons and due to their low price and goodavailability [9,12], iron-based alloys are receiving the most attention aspotential candidates to replace Co-based alloys.

During service of diamond tools, not only the grit, but also themetal matrix has to withstand abrasion and erosion. Therefore,attempts have been made to improve the erosion and wear resistanceby adding hard particles to the matrix [10,13]. For these composites,the interfacial bonding between the reinforcement and the matrix isone of the primary effects influencing the cutting efficiency and thelife time of the tool. If the particle–matrix interface bonding is weakthe hard particles are easily pulled out and may act as sites of internalstress concentration, causing the initiation of cracks which propagatethroughout the matrix.

Many composites based on SiC have been successfully produced,such as Ni–Fe/SiC [14], magnesium alloy/SiC [15], Al/SiC [16]. In thispaper, SiC reinforced Fe–Cu–Sn alloy was developed as a bonding

matrix for fabrication of diamond tools. SiC, as the hard phase, wasselected to improve the wear resistance of the iron-based bond.However, SiC is not stable when in contact with Cu at elevatedtemperatures. Under these conditions, silicon will became partiallydissolved in Cu, leaving pure carbon at Cu/SiC interface [17]. Thecarbon left on the surface of SiC is not wettable by copper [18]. SiCparticles are also destroyed by reactions with Fe and Sn [19]. Thesefactors limit the reinforcing function of SiC particles. Coating SiC withproper metal layer before applying it in Fe–Cu–Sn alloy is one of thepossible ways to overcome the instability of SiC. Titanium can reactwith SiC [20] and the alloy matrix [18] to form TiC and intermetalliccompounds, respectively. Ti coatings on the surface of SiC particleswere therefore developed and the effects of Ti coatings on SiCreinforced iron alloy composites were investigated.

2. Material and methods

The iron-based matrix was composed of commercial grades of65 wt.% Fe, 30 wt.% Cu, and 5 wt.% Sn powders with an averageparticle size of 2−15 μm. Four commercial grade SiC particles withsizes of 28, 40, 88, and 350 μm were used as the wear controller.

Ti-coated SiC particles were prepared by one pot reaction in avacuum reactor. Deposition parameters of are shown in Fig. 1. SiC gritswere mixed with TiCl3 and TiH2 in the vacuum reactor under 10−3 Pafor 1 h, and then heated to 800 °C for 2 h. During heating, TiCl3 wasvolatilized and TiH2 was decomposed according to reactions (1) and(2). A uniform gaseous environment surrounding the SiC grit wasformed which resulted in an increase of the total pressure (1−10 Pa).Afterwards, Ti atoms formed by reduction of TiCl3 by H2 weredeposited on the grit surface according to reaction (3). Afterdeposition, the residual gases were pumped outside and the pressure

Fig. 1. Parameters of Ti deposition process.

4628 Y.H. Wang et al. / Surface & Coatings Technology 205 (2011) 4627–4631

recovered to 10−3 Pa. The samples were then allowed to cool to roomtemperature in the furnace.

TiCl3ðsÞ→TiCl3ðgÞ ð1Þ

TiH2ðsÞ→TiðsÞ þ H2ðgÞ ð2Þ

2TiCl3ðgÞ þ 3H2ðgÞ→2TiðsÞ þ 6HClðgÞ ð3Þ

The surface morphology and the layer structure Ti-coated SiC wascharacterized using a S4800 field emission scanning electronmicroscope (SEM) or/and an energy-dispersive spectroscope (EDS)line scan. The phase structure of Ti-coated SiC was identified bymeansof X-ray diffraction (XRD). The measurements were carried out usinga D/Max-2500pc diffractometer equipped with a standard Cu-Karadiation source employing a step size of 0.02° in 2θ.

Fe–Cu–Sn alloy powder and the SiC particles were mixedthoroughly for 4 h and hot-pressed into a cuboid form of dimensions:40×8×2.5 mm, in a graphite mold at 820 °C and 90 MPa for 2 min.The variables in the preparation of the composites were: Two types ofSiC particles, Ti coated or bare particles; SiC content, 2, 4, 6, 8, 10 or12 wt.%; and four different SiC sizes, 350, 88, 40, or 28 μm.

The bending strength for each sample was tested by using a three-point bending test, which was conducted on a DKZ-5000 testingmachine. The fracture surface morphology was examined using SEM.

The Scheme for wear measurement is shown in Fig. 2. A vitrifiedalumina wheel of 250 mm diameter and 24.5 mm width was used inthe grinding process. The peripheral speed of the wheel, time ofgrinding, and interface force between matrix and wheel were fixed at2850 r/min, 60 s and 15 N, respectively. No coolant was used in thegrinding. The weight of the samples was measured before and afterthe wear test. Weight loss per surface area was determined accordingto Eq. (4):

ΔM =mi−mf

St× 100 ð4Þ

where mi is the initial weight (before the wear test), mf is the finalweight (after the wear test), S is the contact surface area between thesample and the wheel, and t is time of grinding. The grinding surfacemorphology was also examined by SEM.

Grinding wheel

SiC compositeF

Vs

Fig. 2. Scheme for wear resistant measurement.

3. Results and discussion

3.1. Characteristics of Ti-coated SiC

The morphologies of uncoated and Ti-coated SiC particles areshown in Fig. 3(a) and (b), respectively. A distinct Ti coating could befound on the surface of SiC from the SEM image of Fig. 3(b). Moreover,as shown in the XRD pattern of Fig. 4. The coating layer of the SiCparticle consisted of Ti5Si3, TiC, and Ti. The presence of Ti5Si3 and TiCindicated that interaction occurred during deposition, which wascorrelated to the inter-diffusion of Si, C and Ti atoms. Since a smalleratomic radius results in a faster diffusion rate, the diffusion rate isCNSiNTi and C atoms diffuse over long-distances more readily than Siatoms. Thus TiC was adjacent to the Ti side and Ti5Si3 was closer to theSiC side, as shown in Fig. 5. The coating layer was composed of SiC/Ti5Si3/TiC/Ti in sequence from the SiC terminal to the growing surface.This scheme of diffusion reaction of Ti coating and SiC grit wasconfirmed by the element concentration of Ti, Si, and C across thecross section of the Ti coating, as shown in Fig. 6(a). From the coatingto the inner particle, the Ti concentration decreased and the Siconcentration increased. So the inner layer, as shown in Fig. 6(b), wasTi5Si3 and the top layer was Ti. The layer structure was confirmedagain in a sequence of Ti5Si3/TiC/Ti.

The layer structure had two advantages of offering goodtransitions of chemical bonds and thermal expansion coefficients.Firstly, the formation of Ti5Si3 and TiC as the interfacial productsindicated the chemical bonding between SiC particle and the Ticoating. The outside Ti layer is beneficial to good bonding between thecoated particles and the metal matrix. In addition, the thermalstresses on the interface of the SiC/matrix were weakened due to thebuffer layer, whose thermal expansion coefficient lies between thoseof SiC and pure Ti.

3.2. Effect of Ti coating on properties of the composites

Fig. 7 shows the bending strength and the wear resistance of thematrix with uncoated SiC (350 μm) of variable contents. It can be seenthat the SiC addition improvedwear resistance of the matrix, as only a

Fig. 3. SEM images of (a) uncoated SiC and (b) Ti-coated SiC.

Fig. 4. XRD pattern of Ti-coated SiC.

Fig. 6. (a) EDS line scan across the cross-section of Ti-coated SiC; (b) magnifying of (a).

1.4

1.6

1.8

800

900

/ MPa

mg/

mm

2 s Mass Loss IntensityBending strength

•

Δ

4629Y.H. Wang et al. / Surface & Coatings Technology 205 (2011) 4627–4631

small weight loss intensity (b1.1 mg/mm2s) for all samples occurredand the smallest weight loss intensity value was related to the matrixwith 6 wt.% SiC. The bending strength of the matrix was decreasedwith increasing SiC contents.

As a hard phase, an appropriate SiC content leading to a uniformdistribution and firm inlaying in the matrix were beneficial to theimprovement of the wear resistance. When the addition exceeded6 wt.%, the inferior compatibility of the SiC particles with the matrixallowed the particles to be pulled from the matrix easily which lead toa decrease in wear resistance.

The bending strength is mainly governed by the bonding strengthbetween SiC particles and the matrix [21]. However, the affinity andwetting of particles with the elements in the matrix are poor [1] dueto the difference between the covalent bond of SiC and themetal bondof the matrix. SiC particles can be destroyed by their reactions with Feand Sn [19]. Consequently bending strength decreases with theincreasing SiC content.

The bending strength for the Fe–Cu–Sn alloys with 6 wt.% Ti-coatedand uncoated SiC is shown in Fig. 8. It can be seen that the bendingstrength of the compositeswith Ti-coated SiCwas larger than that of thecomposites with un-coated SiC at every SiC size and the increment islarger for alloyswith smaller SiC size. Thewear resistance for the Fe–Cu–Sn alloys with 6 wt.% Ti-coated and uncoated SiC is shown in Fig. 9. Anincrease in wear resistance could be found for the alloys with Ti-coatedSiC particles, compared to that of the alloys with uncoated SiC particles.

As was alreadymentioned, the affinity andwetting of uncoated SiCparticles with the elements in the matrix were poor. Moreover, theuncoated grits can react with the elements in the matrix. The smallersize particles have a larger surface area and a more severe reactionwith the matrix. Consequently, the bending strength of the compositewith uncoated SiC grits decreased with the reduced particle size. Ticoating on the SiC grit provided a barrier layer and excellent bondingbetween SiC particles and the metal matrix. The bending strengthincreased in relation to the particle size: the smaller the size, thelarger the increase.

SiC

Ti

TiCTi5Si3

Si

C

Ti

ing

CoatingCoatingSiC

Fig. 5. Scheme of diffusion reaction of Ti coating and SiC grit.

3.3. Calculation of the interfacial bonding strength

From the above discussion, it is shown that, Ti coating has an effecton realizing the strong bonding between SiC grits and the metal bondmatrix. According to the rule of mixtures, the bending strength of theSiC/composite, σC, is defined as

σC = σB 1−SSiCð Þ + σSiCSSiC ð5Þ

The section density of SiC grits can be expressed as

SSiC =

ffiffiffiffiffiffiφ23

qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi1−φð Þ23

q+

ffiffiffiffiffiffiφ23

q ð6Þ

Here, σB and σSiC are the bending strengths of the pure bondmatrix and the SiC particle, respectively. φ is the volume

0 2 4 6 8 10 120.2

0.4

0.6

0.8

1.0

1.2

400

500

600

700

Ben

ding

str

engt

h

Mas

s lo

ss I

nten

sity

/

SiC contents/ wt.%

Fig. 7. Wear resistance and bending strength of the composite with uncoated SiC ofvariable contents.

Fig. 8. Bending strength of the composites with 6 wt.% Ti-coated and uncoated SiCparticles.

4630 Y.H. Wang et al. / Surface & Coatings Technology 205 (2011) 4627–4631

concentration of SiC grits in the composite and was calculated byEq.(7):

φ =VSiC

VSiC + VBond=

VSiC

VSiC + VFe + VCu + VSnð Þ

=mSiC

�ρSiC

mSiC=ρSiC + 1−mSiCð Þ 65%�ρFe

+ 30%�ρCu

+ 5%�ρSn

� �

ð7Þ

where VSiC and VBond is the volume fraction of the SiC particles and thebondmatrix, respectively. VFe, VCu, and VSn is the volume fraction of Fe,Cu, and Sn, respectively. mSiC is the weight percentage of the SiCparticles, ρSiC, ρFe, ρCu, and ρSn is the density of SiC, Fe, Cu, and Sn,respectively.

For the matrixes with 6 wt.% SiC,mSiC, ρSiC, ρFe, ρCu, and ρSn is takento be 6%, 3.25, 7.86, 8.96, and 7.3 g/cm3, respectively. Substitutingthese parameters into Eq. (7), the calculated volume concentration φis 13.8%, and SSiC can show to be equal to 0.228.

Because the facture path usually proceeds on the SiC/matrixinterface and it is curve, Eq. (5) could be instead of:

σC = σB 1−SSiCð Þ + σSiC−BkSSiC ð8Þ

where σSiC-B is the bonding strength between the grits and the purebond matrix, k is a coefficient, 1bkb2, depending on the morphologyand the granularity of grits, the component of the metal bond, as well

Fig. 9. Wear resistance of the composite with 6 wt.% Ti-coated and uncoated SiCparticles.

as the sintering process. Considering all the impact factors mentionedabove, the coefficient k is selected to be 1.5.

From Eq. (8)

σSiC−B =σC−σB 1−SSiCð Þ

kSSiCð9Þ

Similarly, the bonding strength between the Ti-coated grits andthe pure bond matrix, σTi–SiC–B, is:

σTi−SiC−B =σC Ti−SiCð Þ−σB 1−STi−SiCð Þ

kSTi−SiCð10Þ

where σC(Ti–SiC) is the bending strengths of the composites with Ti-coated SiC grits.

Consequently, the increase of interface bonding strength Δσbetween the grits and the metal bond caused by Ti coating isexpressed as:

Δσ = σTi−SiC−B−σSiC−B =σC Ti−SiCð Þ−σC

kSSiCð11Þ

According to the values of Fig. 9, the calculated increase of theinterface bonding strengths between SiC and the matrix are148.3 MPa (350 μm), 385.1 MPa (88 μm), 407.2 MPa (40 μm), and426.1 MPa (28 μm) owing to the Ti coating, as shown in Fig. 10. As aceramic phase, SiC is brittle and has high strength. During deforma-tion, two types of the microcracks will be initiated by the SiC particles.First, if the interfacial cohesion between the SiC particles andmatrix isstrong, the SiC particles will fracture to nucleate microcracks whenthe local strain and dislocation density reach the critical values by thehigh stress concentration. Second, if the interfacial cohesion betweenthe SiC particles and matrix is weak, decohesion between the SiCparticles and matrix will happen to nucleate microcracks before theSiC particles are fractured. Thus, a strong interfacial cohesion canimprove the strength of the composites (Fig. 8) since weak interfaceswill nucleate microcracks at a rather low external applied stress[22-26].

When Ti-coated SiC grits were incorporated into the bond matrix,the coating has two functions: one is forming the interface (Ti5Si3,TiC) to realize the chemical bond with the grit; the other is providingmetal-metal type of bonding between coated particles and metalmatrix. The excellent bonding strength between the bond and the Ti-coated particles relieved the dissevering effect. Since the smalleruncoated particles have the more serious dissevering effect to thematrix, the contribution of Ti coating to the bonding strength

Fig. 10. Increment of bonding strength between the bond and the Ti-coated SiCparticles.

4631Y.H. Wang et al. / Surface & Coatings Technology 205 (2011) 4627–4631

increased with a decrease SiC particle size. Consequently, theincrement of interface bonding strength increased with a decreaseTi-coated particle size.

4. Conclusions

An iron-based alloy containing 65 wt.% Fe, 30 wt.% Cu and 5 wt.%Sn was designed as a metal bond for diamond grinding wheels in thisstudy. Incorporation of SiC particles into Fe–Cu–Sn matrix couldimprove the wear resistance of the bond matrix, but the bendingstrength of the matrix decreased with increasing SiC content or withdecreasing SiC particles size because of the poor wetting and bondingof SiC and the matrix. Ti coating of the SiC surface improved theinterface bonding strength between the SiC particle and the matrix,and consequently increased the bending strength and wear resistanceof the composite. The increase of the interface bonding strength wasgreater with reducing particle size.

Acknowledgements

The authors gratefully acknowledge the support of the NationalNatural Science Foundation of China (Grant Nos. 50972125 and50872119), Natural Science Foundation of Hebei Province (No.E2010001187), and Element Six Ltd.

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