Tribology International 34 (2001) 8993www.elsevier.com/locate/triboint

Some aspects of solid particle erosion of cermetsI. Hussainova *

Department of Materials Technology, Tallinn Technical University, Ehitajate tee 5, 19806, Tallinn, EstoniaReceived 27 September 2000; accepted 28 October 2000

Abstract

The erosive wear resistance of cermets with different composition, structure and properties has been investigated. It was shownthat the erosive wear resistance of cermets can not be estimated only by hardness, characterizing resistance to penetration. Thedifferences in wear resistance between cermet materials with equal hardness levels can be attributed to differences in their resistanceto fracture and fracture toughness. The mechanism of erosion depends first on testing conditions. The present paper discusses somefeatures of the material removal process during particle-wall collision. Systematic studies of the influence of the impact variableson the collision process have been carried out. 2001 Elsevier Science Ltd. All rights reserved.

Keywords: Ceramic metal composites; Hard metals; Solid particle impact; Wear resistance

1. Introduction

Widespread use of cermets as materials for workingelements of various equipment and machine parts maybe attributed to their unique combination of desirableproperties such as high hardness, strength, stiffness andwear resistance.

Because of a lack of common information about dif-ferent types and grades of cermets, it is of interest andimportance to test the materials and to identify their wearbehaviour under different conditions to choose the opti-mum metalmatrix composite. WC-based cobalt-bondedhard metals are most widely used because of their excel-lent wear resistancestrength combination. Shortage oftungsten and cobalt and their poor corrosion resistancein corrosive mediums and elevated temperatures restrictsapplication of these hard metals. For this reason, the so-called tungsten-free hard metals have been developedand adopted in industry. Nickel and iron, or their alloys,are used as binders in these metalmatrix composites.The most widely known among tungsten-free cermetsare based on TiC and Cr3C2 cemented with Ni and Moalloys. Steel-bonded cermets form a special groupamong hard metals. They are of interest because theyare inexpensive and heat treatable.

* Tel.: +372-620-3303; fax: +372-620-3196.E-mail address: irina@meo.ttu.ee (I. Hussainova).

0301-679X/01/$ - see front matter 2001 Elsevier Science Ltd. All rights reserved.PII: S0301- 67 9X( 00 )0 0140-7

2. Test materials and experimental details

The cobalt content of the WCCo hard metals was 815 mass %. The content of TiC in TiC-based cermetswas 4080 mass %. Four grades of these materials con-tained bonds with different composition and structure.All TiC-based cermets were sintered in vacuum. Theaverage grain size of carbides was, depending on compo-sition, 2.02.7 m m, porosity 0.10.2 vol.%. In Cr3C2Nicomposites the content of metal binder was 1530 mass%. The test materials and their properties are given inTable 1.

All materials were produced at the Institute ofMaterials Technology, TTU.

The abrasive particles used in this work were siliconcarbide, as used in most standard erosion tests, and sil-ica, the most common naturally occurring erodent. Theparticles were sieved into the required size fractionsbefore erosion testing. The size of erodent particles was0.10.3 mm. The mechanical properties of abrasive par-ticles are listed in Table 2 and SEM micrographs of usederodents are shown in Fig. 1. The silicon carbide is moreangular, with shaper profiles, while the silica is ratherrounded.

A centrifugal four-channel device was used to acceler-ate the abrasive particles for the studies of erosion resist-ance. Specimens with dimensions 20 12 5 mm weretested. The test surface was polished and then cleanedin acetone prior to impacting it with the projectiles. An

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Table 1Composition and properties of cermets

Grade Carbide content, Composition and Vickers hardness Density r, kg m-3 Modulus of elasticity(mass%) structure of binder number HV E, GPa

BK8 92 WC Co 1350 14500 650BK15 85 WC Co 1200 13900 560TZC40 40 TiC FeCr9Si1.5 1150 6100 300TZC60 60 TiC FeCr7Si1.5 1360 5800 380TH20A 80 TiC Ni13 Mo7 1378 5500 400TH40A 60 TiC Ni26 Mo14 1190 5770 380K31 85 Cr3C2 Ni 1410 6970 340KE3 70 Cr3C2 Ni 980 7190 320

Table 2Mechanical properties of abrasive particles

Erodent Hardness HV Density r, kg Fracturem23 toughness Kc,

MPa m1/2

SiC 3000 3300 3.52SiO2 1100 2150 1.6

accuracy of 0.1 mg was obtained for the target massloss measurements.

Investigations of the steady state erosion rate weremade as a function of the impact velocity and impactangle. Wear conditions were: impact angle, deg: 30, 45,60, 75, 90 and impact velocity, ms21: 31, 61, 80 . Thenumber of test samples of each cermet grades was four.

The erosion rate was determined as volume loss ofthe target sample per mass of erodent particles (mm3/kg) to facilitate the comparison of target materials withdifferent densities.

Fig. 1. Erodents used in this study: (a) - silicon carbide; (b) - silica.

3. Results

3.1. Steady state erosion rates of the test materials

Figs. 2 and 3 show the steady state erosion rates ofthe test materials impacting by silica and SiC particlesat velocity of 61 ms21 under an initial angle of 75 .

In all tests, BK8 clearly outperforms the othermaterials. The durability of TiC and Cr3C2-based cer-mets compares unfavourably with that of WC- basedones and the wear resistance difference between differ-ent families of cermets at the same hardness reaches400%. Remarkable differences between erosion ratescould not be explained by differences in hardness orshear strength. Therefore, for wear resistance evaluation,hardness is only useful as a first approximation. It mustbe noted that the hardness values are based on low strainrate hardness measurements, but the hardness valuesactually relevant to the impact conditions occurring inerosion may be somewhat different.

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Fig. 2. Steady state erosion rates of the test materials. Erodent:- sil-ica; velocity: 61 ms21; angle: 75 .

Fig. 3. Steady state erosion rates of the materials investigated. Abras-ive: SiC; velocity: 61 ms21; angle: 75 .

The mechanism of erosion depends firstly on the testconditions [1]. At low kinetic energy of abrasive par-ticles, selective nature of erosion prevails. In the con-ditions of high kinetic energy of abrasive particles a low-cycle fatigue failure of the carbide matrix and large car-bide grains and microcutting occur depending on theratio of material and abrasive hardness.

During erosion processes the fracture of material startslocally and in most cases, in the binder phase [1]. Car-bide grains lose their protective binder and the erodedsurface is almost entirely covered with the exposed car-bides. If material hardness exceeds that of abrasive, theerodent particles are not able to cause plastic flow inhard target and selective nature of erosion prevails. Thedegree of elastic penetration and therefore energy trans-mitted to a surface depends on elastic modulus and ifthe latter is high, negligible elastic penetration occurs.Under these conditions the impact of abrasive particlesmay cause a low-cycle fatigue failure of the carbidematrix and carbide grains [2]. Thus, the elastic modulusis an even more important parameter for wear resistancethan hardness. Among materials investigated, WC-8%Coalloy has the highest modulus of elasticity.

If the hardness of an abrasive exceeds that of cermet,

the following processes take place: penetration of abras-ive into the material surface, microcutting or plowing,failure of large carbide grains, resulting in the detach-ment of small chips. Under these conditions there is nogreat difference between cermet grades. Since the ero-sion of brittle grains is primarily via a mechanisminvolving the initiation and propagation of microcracksone expects that the fracture toughness of the materialwill affect the erosion rate. The low toughness of mosthard constituents resulted in a loss of erosion resistancethat explained the poor performance of brittle Cr3C2-based cermets and the possible benefit of the tougherWC-based ones. In contrast to the brittle TiC and Cr3C2grains, tungsten carbide grains is tough enough to with-stand contacts with high amounts of dissipated energywithout microfracture. The mechanical and wearingproperties of each individual grain and the boundariesbetween adjacent grains are therefore important for theoverall performance of the carbide materials.

The specific wear behaviour was shown by Cr3C2-based cermets. If the erodent is much harder that thetarget material, crack initiation is inevitable and crackpropagation is the rate controlling factor. The grade withthe content of brittle Cr3C2 grains of 85 mass % has apoor wear resistance that may be attributed to its lowfracture toughness of about 3 MPa m1/2 and weak bond-ing between the grains. In the case of Habrasive, Htarget,K31 grade has enough good wear resistance that may beattributed to its high hardness and therefore high resist-ance to plastic penetration. It can be seen that the lateralcracking mechanism of erosion [3], operating with sili-con carbide erodent, leads to much higher rates of ero-sion than the minor chipping mechanism which occursin the case of silica. From this result, it can be concludedthat in quantifying the erosion resistance of materials foruse in particular environment, care must be taken toensure that the test and service environments producethe same erosion mechanism. Comparative data obtainedfor materials eroded by silicon carbide particles, wherethe brittle fracture mechanisms dominates, may havelittle relevance to the comparative erosion resistance ofthe same materials when eroded with silica in an indus-trial environment if erosion then occurs by a completelydifferent and less severe mechanism. If different mech-anisms operate under service conditions from those dom-inating during testing, misleading conclusions will bedrawn from the test results.

3.2. The effect of impact angle on erosion rate

Figs. 46 illustrate the erosion rates of three cermetgrades plotted against the angle of impact.

As can be seen, there are some differences in wearbehaviour of materials tested. TiC- based cermets exhibitthe maximum erosion rate at impact angle of 75 , butCr3C2-based ones have poorer erosion resistance at 90 .

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Fig. 4. Effect of angle on the erosion rate of TiC-based cermets.Erodent: silica; velocity: 61 ms21.

Fig. 5. Effect of angle on the erosion rate of Cr2C3-based cermets.Erodent: silica; velocity: 61 ms21.

Fig. 6. Effect of angle on the erosion rate of WC-Co cermets. Erod-ent: silica; velocity: 61 ms21.

This is closer in behaviour to brittle ceramic materials.Erosion of brittle materials occurs by the propagationand intersection of cracks caused by impacting particles.At an impact angle of 90 the stress is remarkably higherthan at an impact angle of 30 and, as a result, wear ratesare higher. The two tungsten carbidecobalt cermets, onthe other hand, exhibit a maximum erosion rate at 60 that is closer to behaviour of ductile materials, whichhave a maximum between 30 and 60 impact angle.There are clearly two competing mechanisms of erosion,one responsible for the loss of the softer binder and theother leading to the loss of the brittle WC, which wouldexplain the shift in maximum from 30 to 60 .

Fig. 7. Comparative evaluation of the relative performance ofmaterials tested. Erodent: silica; impact velocity: 31 ms21.

Fig. 8. Comparative evaluation of the relative performance ofmaterials tested. Erodent: silica; impact velocity: 80 ms21.

Fig. 9. Comparative evaluation of the relative performance ofmaterials tested. Erodent: SiC; impact velocity: 31 ms21.

A comparative evaluation of the relative performanceof the materials tested (Figs. 710) shows that erosionresistance of the relatively harder alloy K31 comparesunfavourably with that of others grades. The intergranu-

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Fig. 10. Comparative evaluation of the relative performance ofmaterials tested. Erodent: SiC; impact velocity: 80 ms21.

lar cracking appearing in the abrasion tests of Cr3C2-basecermets [3] may be explained by weak bonding betweencarbide grains and weak intercarbide boundaries.

The mechanical and wearing properties of each indi-vidual carbide grain and the boundaries between adjac-ent grains and binder are therefore important for theoverall performance of the cermet materials. The tung-sten carbide based materials do not exhibit any transitioninto a high wear level that can be attributed to the hightoughness (about 15 Mpa m1/2) and the ability of carbidegrains to withstand loads without microfracture [4]. Athigh kinetic energy, the difference between differentgrades of cermets is visible (Fig. 8).

The brittle materials are very sensitive to the hardnessand fragility of erodents [5]. Thus, the erosion rate ishigher for materials tested if the erodent hardness isgreater than that of the target.

If the impact angle is shallow, the erosion resistanceof relatively brittle TiC-based alloys can be comparedfavourably with one of WC-based cermets (for almostequal hardness of alloys). TiC-based cermets are pro-spective first in working conditions with modest kineticenergy of abrasive and at impact angle less than 60 .The higher the angle of collision, the more the advantageof WC-based grades over others is.

With the SiC abrasives, at low kinetic energy of theparticles, WC-based and TiC-based cermets show thesimilar wear rates, Cr3C2 shows the highest wear rate,especially at collision angle of 90 . TH20A alloy maybe an attractive material for the erosive wear problem

provided the hardness of erodent is greater than that oftarget at impact angle lower than 75 .

4. Conclusions

Erodent particles of different material can cause dif-ferent erosion rates and mechanisms. All materials showa significantly lower wear rate when abraded with silicathan with SiC. This may be explained by the influenceof hardness of the abrading particles compared to thetest material. The harder silicon carbide particles causeplastic indentation and lateral cracking, whereas thesofter silica particles cause minor chipping on a muchfiner scale that that of particle contact area. The mainmechanism of wear is a low-cycle fatigue.

Based on the erosion results presented for three typesof cermets, a maximum volume loss occurred for (1)WCCo cermets at an impact angle of 60 ; (2) TiC-based cermets at an impact angle of 75 ; (3) Cr3C2-(Ni,Mo) cermets at an impact angle of 90 .

The maximum position depends on the materialresponse to impact.

In the present study, the relative ranking of thematerials investigated with respect to erosion rate couldbe explained by their different fracture toughness valuesand modulus of elasticity, whereas the hardness seemsto be of minor importance.

The TiC-based cermets may be an alternative to WC-based ones in the case of low impact angles and lowkinetic energy of erodents. The TiC-based alloys maybe the attractive candidate material for the erosive wearproblem provided the hardness of the erodent is greaterthan that of the target.

References

[1] Reshetnyak H, Kubarsepp J. Mechanical properties of hard metalsand their erosive wear resistance. Wear 1994;177:185.

[2] Hussainova I, Kubarsepp J, Shcheglov I. Investigation of impactof solid particles against hardmetal and cermet targets. TribologyIntern 1999;32:337.

[3] Pirso J, Viljus M, Kubarsepp J. Abrasive erosion of chromiumcarbide based cermets. Proc of 9th Symposium on TribologyNORDTRIB 2000, Porvoo 2000;3:959.

[4] Engqvist H, Axen N, Hogmark S. Tribological properties of abinderless carbide. Wear 1999;232:157.

[5] Ball A, Feng Z. The erosion of four materials using seven erod-entstoward an understanding. Wear 1999;233:233.