Fretting wear of selected ceramics and cermets

Fretting wear of selected ceramics and cermets

Wear, 274 (1994) 47-56 47 Fretting wear of selected ceramics and cermets P.Q. Campbell, J-P. Celis, J.R. Roos and 0. Van Der Biest Department of Met...

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Wear, 274 (1994) 47-56

47

Fretting wear of selected ceramics and cermets P.Q. Campbell, J-P. Celis, J.R. Roos and 0. Van Der Biest Department of Metallulurgvand Materials Engineetig,

Katholieke Universiteit Leuvert, B-3001 Leuven (Belgium)

(Received August 11, 1993; accepted February 3, 1994)

Abstract Fretting experiments were performed on sialon ceramic, zirconia-toughened alumina ceramic, titanium carbonitride cermet and tungsten carbide-cobalt cemented carbide materials to determine their relative wear performance when tested against Cr-steel counterbodies. All of the materials, with the exception of the titanium carbonitride cermet, display fretting steady state coefficient of friction values similar to that of steel on steel. The wear behavior of the sialon/steel couple appears to involve transfer layers, indicating that tribochemical interactions play an important role in this case. Roth the zirconia-toughened alumina/steel and tungsten carbide-cobalt cemented carbide/steel couples undergo abrasive wear during fretting testing, though the tungsten carbide-cobalt cemented carbide is worn to a lesser degree than the zirconia-toughened alumina. The titanium car&nitride cermetlsteel couple, on the other hand, undergoes a special abrasive wear mechanism in which hard particles from the cermet embed themselves in the soft steel, where they preferentiaily abrade the cermet while protecting the steel.

1. Intmduction 1.1. Fretting Fretting refers to any situation in which the contacts between materials are subjected to a low amplitude oscillatory sliding motion. Fretting wear often occurs on surfaces which are intended to be fixed in relation to each other, but still experience a small oscillatory relative movement. Fretting often takes place in hubs and disks press-fitted to rotating shafts, in riveted and bolted joints, between the strands of wire ropes, and between the rolling elements and their tracks in stationary ball and roller faces [l]. The displacement amplitudes encountered in fretting are smaller than those of reciprocating sliding. Experimental data [2] suggest that the transition to reciprocating sliding occurs in the interval 150-300 pm. This means that contact is maintained over most of the tribosurface during fretting. As a result, much of the wear debris produced by fretting remains trapped at the interface, which can cause seizure in items such as flexible couplings [3]. This debris often includes a large amount of oxides, and hence, the term fretting cormsion is often applied. Another important aspect of fretting is the development of fatigue cracks in the damaged region, which reduces the fatigue strength of a cyclically loaded component. When the cyclic loading that is responsible for the fretting wear is linked to the cyclic loading

causing the fatigue crack to grow, the greatest reduction in fatigue strength is seen. When such a mechanism results in a fatigue failure, the term fretting fatigue is often applied [I]. 1.2. gateway Ceramics have a unique set of properties that make them suitable for a variety of tribological applications, such as those involving cutting tools [4], bearings, turbine blades, and face seals. The properties that make ceramics suitable candidates for these applications include a high resistance to wear, a high hardness, a high resistance to plastic deformation, a high compressive strength, retention of properties at elevated temperatures and chemical stability. Three general classes of ceramic materials that are available today are pure oxides, mixed ceramics and nitride ceramics [5]. Alumina (AlzO,) is the predominant ~nstituent in the oxide ceramics, and silicon is a primary component in the nitride ceramics. Thus it is convenient to divide the ceramics into classes based on alumina and silicon nitride. Alumina-based ceramics are often reinforced by materials such as zirconia (ZrOz), titanium carbide (Tic), titanium nitride (TiN) and silicon carbide (SIC). Zirconia additions to alumina ceramics reduce their susceptibility to fracture by retarding crack propagation by transforming from a metastabie state to a stable state when a crack is initiated. The volume change associated with this transfo~ation causes compressive

0043-1648194/$07.00 0 1994 Efsevier Science S.A. All rights reserved SSDI 0043-1648(94)06436-L

48

P.Q. ~~rnp~l~ et al. f Fretting wear of ceramics and cermets

stresses to develop at the crack tip, impeding crack propagation [5]. The zirconia-toughened alumina ceramics are in the class of pure oxide ceramics. The major disadvantage of pure oxide ceramics is their low thermal shock resistance, which is a result of their low thermal conductor. The addition of titanium carbide or titanium nitride to alumina can increase the thermal conductivity of the ceramic, thus improving the thermal shock resistance. The alumina ceramics, with TiC and TiN additions, are classed as mixed ceramics. Another advantage of mixed ceramics over pure oxide ceramics is that they are harder and maintain their hardness better at high temperatures [S]. Alumina-based materials are employed in various tribological applications such as in check valves, cutting tools, pump seal rings, thread guides and bearings. Alumina is also utilized in prosthetic articulating joints because of its low weight, excellent wear resistance and biocompatibility [7]. Silicon nitride-based ceramics often contain yttria (Y203), which yields higher bend strengths at higher temperatures, and which generates a more compact sinter and uniform structure. Sialon ceramics are solid solutions of Si, Al, 0 and N with the P-S&N, crystal structure. The sialons are usually produced by making additions of AlN, MgO, BeO, Y20, or other metal oxides to silicon nitride. Additions of metal oxides cause a distortion of the lattice, and hence, these materials are sometimes referred to as @-S&N, [7]. These sialons have similar physical properties to /3-S&N,, but exhibit better chemical properties because of the chemical substitutions involved in the process. The importance of the chemical substitution on the properties is manifested mainly in two ways: (1) the greater degree of control over the oxygen level by its substitution in /3’ crystals improves the properties of creep resistance and resistance to subcritical crack growth and (2) the substituted A13’ ions tend to offset the ~cosi~-reducing tendency of impurity cations (e.g. Mg and Ca) within the silicate glass protective oxidation layer 181. The important point to note here is that these chemical substitutions allow the intergranular phase to be absorbed, improving some of the mechanical properties of the ceramic. Silicon nitride-based materials are used in tribological applications such as seals, pump sleeves, bushings, valve components and cutting tools. Silicon nitride is also one of the most promising candidates for high temperature bearing applications 171. combination of Cermets are “a heterogeneous metal(s) or alloy(s) with one or more ceramic phases in which the latter constitutes approximately 15 to 85 percent by volume and in which there is relatively little solubili~ between metallic and ceramic phases at the preparation temperature” [93. Cermets are engineered to balance hardness and toughness. In other words, the hard particles provide a high hardness and wear

resistance, and the metallic binder provides for ductility and toughness. The first cermets were the tungsten carbidexobalt cemented carbides, which have been used for cutting tool applications, metal forming applications, seal rings, valve components and bearings. Since then, cerrnets containing hard particles such as tit~ium carbide (Tic), tit~ium nitride (TiN), titanium carbonitride (Ti(C,N)), tantalum carbide (TaC), molybdenum carbide (Mo2C), boron carbide (BJ) and chromium carbide (Cr3CJ in suitable metallic binders such as iron, nickel, cobalt, molybdenum and chromium have been developed. Cermets based on titanium carbide and titanium carbonitride exhibit an enhanced strength, which makes them approp~ate for high speed machining operations. This enhanced strength is based on a greatly improved bond between the hard carbide grains and the binder metal, which stems from a miscibility gap in the quaternary Tic, TiN, MoC and MoN system, which results in a spinodal decomposition into two isostructural phases with intrinsically better wettability to the binder 193,

2. Experimental procedure 2.1. Materials and sample preparation A list of the sintered materials used in this work, along with some of their properties, is given in Table 1. These materials were provided by the C&am&al company in Luxembourg. The sialon material contains 5 wt.% yttria (Y,O,), 4 wt.% alumina (Al,O,) and the remainder is silicon nitride (S&N,& The alumina-based material consists of alumina and 10 wt.% zirconia (ZrO,). The titanium carbonitride cermet is composed of 59.6 wt.% titanium carbonitride (Ti(C, N)), 11.1 wt.% tungsten carbide (WC), 15.2 wt.% molybdenum carbide (Mo,C), 8.7 wt.% cobalt and 5.4 wt.% nickel. The tungsten carbide-cobalt material contains 6 wt.% Co, with the remainder being WC. These materials are ground and polished until they have a surface roughness R, roughly equal to 0.01-0.02 pm. The surface roughness in this case was measured in house with a Taylor-Hobson Form Talysurf, and the measuring parameters were set according to the DIN 4768 standard. The Cr-steel spherical counterbodies comprise SAFZ2080 steel with a hardness of 63-66 HRC,. a diameter of 10 mm and a surface roughness R, =0.02 pm. It is suspected that wear involving the iron oxide third body in the fretting test might be related to steel-on-steel behavior. Thus, fretting tests with steel on steel have also been performed. The flat steel specimens are the German steel designated 42 CrMo 4, which has a hardness of roughly 37 HRC. Before a fretting experiment is started, the ceramic or cermet and the corresponding Cr-steel counterbody are ultrasonically cleaned in acetone. At the

P.Q. Campbell et al. I Fretting wear of ceramics and cermets TABLE

1. Some properties

of the ceramics

Material

Sialon A1203-Zr02 Ti(C,N) cermet WC-Co

and cermets

used in this work

Density

Hardness

Fracture

(g cmW3)

(WlO)

(MPa)

Toughness (MPa mm)

3.24 4.13 6.38 14.90

1560 1620 1640 1810

1000 610 1900 2200

7.0 3.2 8.3 7.7

low loads used in this testing, contaminant films can affect the wear behavior of the materials [lo]. Hence, the sample preparation was kept consistent throughout the testing so that comparisons can be made. 2.2. Fretting tests A schematic diagram of the fretting apparatus used in the laboratory wear experiments is given in Fig. 1. A ball-on-flat geometry was employed, and the normal load is applied as a dead-weight. The flat sample is mounted on a translation table, which oscillates the sample at the set displacement and desired frequency by means of a stepping motor. The displacement of the sample is monitored with the aid of an inductive displacement transducer, and the friction force is measured with the aid of a piezoelectric transducer attached to the holder that supports the spherical counterbody. These signals are amplified and then displayed on-line through an oscilloscope, and data are acquired by the computer at pre-set numbers of cycles. The acquired data are then analyzed to produce fretting loops (friction

MTM

FRETTING

APPARATUS

NORMAL LOAD

I CHARGE AWLIFIER

WUTER

1

Fig. 1. Schematic diagram of the fretting apparatus Department of Metallurgy and Materials Engineering Katholieke Universiteit Leuven.

at the at the

strength

force VS.displacement hysteresis loops) and to determine the coefficient of friction (COF), which is a value averaged over one complete cycle. All tests were performed in ambient air of relative humidity of 50-70% at 22 “C and under unlubricated sliding conditions. The reader is referred to the literature [ll] for further information on this fretting apparatus. 2.3. Equipment for evaluation of fretting wear scam A Rodenstock RM600W-100 profilometer was used to evaluate the geometry and wear volumes of the fretting scars. This apparatus is designed for non-contact measurements between depths of 0.01 and 600 pm, and it is based on the focusing of an infrared laser beam on the surface of the material. A spot with a diameter of 1 or 2 pm is created, and the beam is reflected back to a focus detector. During a measurement, the beam is moved smoothly over the sample and the measured values form a contour profile. It is also possible to scan several neighboring profiles in a grid-like form with the aid of an X-Y traversing table. The measured values then form a three-dimensional contour profile which can be output in various graphic displays such as a three-dimensional projection, contour lines, and surface levels. The characterization of the wear tracks was performed with the aid of scanning electron microscopes (a Philips 515 SEM and a JEOL SUPERPROBE 733 SEM) equipped with energy-dispersive X-ray spectroscopy systems which aid in identifying the elements (Z>Z,,) present in the materials under study. The JEOL SUPERPROBE 733 SEM is also equipped with a wavelength-dispersive spectroscopy system, which was used for detecting the presence of oxides.

3. Experimental

PIEZOELECTRIC TRANSOUCER

49

results

3.1. Determination of fretting regimes It has been shown in the literature [2] that the contact conditions in fretting change with increasing displacement amplitude. The sphere-on-flat geometry used in this study results in a circular contact area, and under elastic conditions the contact pressure reaches a maximum at the center of the contact circle and falls to

50

P.Q. Campbell et al. I Fretting wear of ceram& and cermets

zero at the edges. If a small cyclic tangential force is superimposed on the normal load, some microslip can occur between surfaces at the outer edges of the contact circle where the contact pressure, and hence the frictional stress opposing the movement, is lower. In this instance, the contact zone comprises both a central stick area where there is no relative tangential displacement and of a region where there is microslip. As the applied tangential force is increased, the size of the central stick area decreases until slip inevitably takes place over the entire contact region [l]. The applied tangential force can be transformed into a tangential displacement, which is a more convenient measure of the fretting conditions. Using such data, three different fretting regimes have been identified in the literature [Z] employing friction force-displacement hysteresis loops such as those shown in Fig. 2 for a Cr-steel ball sliding over the sialon of this study. The area of each fretting loop represents the energy dissipated through friction. The three regimes are as follows. (4 A stick regime in which the displacement is accommodated by elastic deformation of the asperities and no energy is dissipated, as seen from the closed loop in Fig. 2(a).

FRETTING HYSTERESIS LOOPS FOR SIALON/Cr-STEEL

(cl

B_~~ loo

t%~Uc+oEME% &

Fig. 2. Friction force-displacement loops representing the different fretting regimes for the fretting testing of the sialon against Crsteel under a normal load of 1 N and a frequency of 8 Hz. Note that the displacement is the parameter that is controlled. (a) Stick regime. (b) Stick-slip regime. (c) Gross slip regime.

stick-dip regime in which there is a central stick region, where the corresponding asperities elastically deform, and an outer annular region where rnic~s~i~ occurs. Thus, some of the energy is dissipated through sliding, but part of the displacement is accommodated by the elastic deformation of asperities with a particular time delay, causing a phase shift to be interjected between the friction force and displacement. This phase shift is manifested in the form of an elastic slope on the sides of the loop as seen in Fig. 2(b). Cc)A gross slip regime in which slip occurs throughout the entire contact region and the work done by sliding is irreversibly dispersed via friction. This case is depicted in Fig. 2(c). Since larger wear volumes are associated with the gross slip regime [l], fretting experiments using gross dip conditions were performed in this work so that the analysis of the resulting fretting wear volumes is easier. For more information on fretting wear in general, the reader is referred to the literature [l-3,12-18]. In order to ensure that the experiments were performed in the gross dip regime, fretting maps for the various material combinations were first determined in a cursory fashion. A fretting map is an illustration that portrays the pertinent regimes in two variables, where the regime boundaries represent the critical values for the transition from one regime to another [2]. The points of transition can be determined either by observable changes in scar morphology or from the friction and displacement data. The determination of these maps has been solely based on the fretting loops acquired from tests enduring 250 cycles at a frequency of 8 Hz. Two parameters were varied in this instance: the normal load and the displacement. The displacement was varied from 5 to 100 pm, in increments of 5 pm, and normal loads of 1, 2, 3, 4 and 5 N were used. The transition from the stick regime to the sock-skip regime was determined from visual inspection of the fretting hysteresis loops from the 250th cycle of each experiment. As soon as the loop has “opened up”, it is considered that the transition has been made. It should be mentioned here that this method is somewhat subjective, so care must be taken when utilizing this data. The determination of the transition from stick-slip to gross slip, however, is slightly more complicated. The point of incipient gross slip, in terms of displacement amplitude, can be found from either the friction force-displacement relations or from frictional energy dissipation. When the increasing displacement A has reached the critical gross slip value AcR, there is a drop in the tangential force F, to avalue Fx, a, corresponding to the transition from static to kinetic friction [15]. Hence, the point coinciding with the position (ACR, F,,,,) is classified as a critical transition coordinate.

(b) A

P.Q. Campbell et al. f Fretting wear of ceramics and cemets

It has been found in the literature, however, that the critical transition coordinate is easier to discern with a dissipated energy criterion [lS]. If the dissipated energy E is plotted as a function of displacement A, it generally shows a monotonically increasing pattern like that of Fig. 3, which has been determined from data from experiments dealing with the sialon/Cr-steel couple. Each point on this curve was taken from the fretting loop corresponding to the 250th cycle of fretting under a normal load of 1 N and a frequency of 8 Hz. A major result from the literature [15] is that there is always a sudden increase in the slope dEldA of the energy curve for roughly the same A as the ACRof the force curve described above. Thus, the sudden change in slope at the ACR indicated in Fig. 3 signifies the point of incipient gross slip. This energy criterion has been found in the literature [15] to yield a better agreement between results of experiments performed under the same conditions than the force criterion does. Consequently, the energy criterion was used to determine the points of in~ip~~t gross slip for the fretting maps. An example of a fretting map can be seen in Fig. 4, which was determined from data acquired from sialoni Cr-steel fretting experiments. The boundaries were drawn as best-fit lines through the points determined from experimental data. The correlation coefficient was greater than 0.9 for all cases, indicating that a linear fit is reasonable. Linear behavior is, in fact, expected in many cases in these types of fretting maps [2]. It can be seen in Fig. 4 that a point corresponding to a displacement of 100 pm and a normal load of 1 N is well into thegross slip regime. In fact, these test conditions are well into to the gross slip regime for all of the

3 @

50

z zi 0

0

20

40

60

80

100

DISPLACEMENT (,um) Fig. 3. testing change slip at

Dissipated energy-displacement diagram for the fretting of the sialon against a O-steel counterbody. The rapid in slope signifies the transition from stick-slip to gross the critical displacement for incipient gross slip, AcR.

51

FRETTING MAP FOR SIALON/Cr-STEEL

z;;j

Stick

0

[email protected]/

Gross Slip

20

80

$

100

DISP!?CEMEN~“(~m) Fig. 4. Fretting map for the testing of the sialon against Cr-steel using a frequency of 8 Hz.

material ~mbinations of this work. Hence, these parameters were chosen for the fretting experiments. Finally, it should be noted that changes from one regime to another during the 250 cycles of testing were observed in this work as well as in other work with this fretting apparatus [16]. Also, there are transitions to higher friction forces (or coefficients of friction) as time proceeds, as described later in this paper. Consequently, the fretting maps have only been used as guides to ensure that we are working in the gross slip regime. Examination of the fretting loops in this work confirms this. 3.2. Friction mults The dependence of the coefficient of friction (COF) on the number of cycles for the various materials is shown in Fig. 5. It can be seen that in all cases the COF rises rapidly during the initial running-in period and then levels off at some steady state value for the remainder of time up to 500000 cycles. The steady state value of the COF for each of the materials, with the exception of the titanium carbonitride cermet, is between 0.6 and 0.7. It should be noted that this value of the COF is close to that for the fretting testing of the steel on steel as can be seen in Fig. 5 and also as has been reported in the literature [19]. The steady state value of the COF for the titanium carbonitride cermet, in contrast, varies between 0.4 and 0.5. 3.3. Results on extent of wear The degree of wear is represented in two different ways here: projected maximum depth profiles and wear volumes. The projected maximum depth profiles are employed here to illustrate how a fretting pit grows. These depth contours are found by selecting a position

P.Q. Ca~p~~~ et al. f Fretting wear of ceramicsand

52

P =

1 N, A =

100

km,

f =

cermets

8 Hz 3ALON

$ Oh83’

Cr4lEEL

II P=lN

Cf-STEEL

5

* ” 1”



( 2”

NUMBER

I,

’ ) ’ 3

t

I

I

I,

4

OF CYCLES

I

I

I

5 x105

Fig. 5. Coefficient of friction as a function of number of cycles during the fretting testing of the ceramics, cermets and 42 CrMo 4 steel against a Cr-steel counterbody (Pm load, A = displacement, f = frequency). P = 1 N, A = 100 &m, f = 8 Hz, Counterbody:

Cr-Steel

0

F B n

f=BM

J

-10

-,I/, ,,,,,,,,,,,,“,mi,/

Fig. 7. Fretting wear volumes of the ceramics, cermets and corresponding Cr-steel counterbodies after testing for 500 000 cycles (P E load, A = displacement, f = frequency).

described in the literature 120). It appears that the sialon wears the most against steel of all the materials while the tungsten carbidexobalt cemented carbide wears the least of all the materials. The extent of wear on the zirconia-toughened alumina is also relatively small while the wear on the titanium carbonitride cermet is slightly greater than that of the zirconia-toughened alumina. It is interesting to note that all the materials, with the exception of the titanium carbonitride cermet, have wear volumes less than the corresponding counterbody, as might be expected from their higher hardness. The reason the cermet has a wear volume slightly greater than that of the corresponding counterbody is attributed to a special abrasive wear mechanism that is discussed in Section 4.

-100 0 100 300 -300 -200 200 DISTANCE FROM CENTER OF FRETTING PIT (pm) Fig. 6. Evolution of a fretting pit. The contours represent projected maximum depth profiles extracted from fretting pits produced on the sialon with a Cr-steel counterbody after various cycles of testing (Pm load, A = displacement, f Efrequency).

along the length of pit and then obtaining a depth profile across the width of the pit at that position. The rn~~urn depth found at this position is then recorded and plotted with the rn~urn depth at other positions in the form of Fig. 6. The depth contours in Fig. 6 are shown for fretting pits formed from various numbers of cycles of testing, which indicates how a fretting scar might evolve in a sialon when it is tested against Crsteel. All the materials exhibit a similar growth pattern, but show varying degrees of wear as can be seen from the wear volumes plotted in Fig. 7. The wear volumes of the Cr-steel counterbodies in Fig. 7 were estimated using a spherical cap volume calculation as has been

It has been observed that the fretting testing of the 42 CrMo 4 steel against the Cr-steel produces significant amounts of iron oxide debris, which proceeds to form scratches in the contact zone through an abrasive wear mechanism. Some material transfer of iron also occurs in the contact zone. An example of a typical fretting scar on a sialon after being fretting tested against a Cr-steel counterbody for 500 000 cycles can be seen in Fig. 8(a). It should be noted here that all of the photomicrographs were taken from “unclean” surfaces. In other words, the wear debris has not been removed from the surface to facilitate the examination. There are iron transfer layers on the bottom of the pit, and a close-up view of a section of such a transfer layer can be seen in Fig. 8(b). Transfer layers can also be seen on the wear scar on the corresponding Cr-steel counterbody as shown in Fig. 9. Energy-dispersive analysis of both wear scars indicates that these transfer layers are a mixture of

P.Q. Campbell et al. / Fretting wear of ceramics and cennets

I

100

pm

53

ticles from both the sialon and Cr-steel counterbody. Wavelength-dispersive analysis of the agglomerated debris suggests that the bulk of the debris is composed of iron oxides, which agrees with their reddish-brown appearance when observed under an optical microscope. An example of a fretting wear pit on the zirconiatoughened alumina after 500 000 cycles of fretting testing against a Cr-steel counterbody can be seen in Fig. 10. There appear to be some small scratches on the bottom of the pit, and some compacted and agglomerated debris, composed mainly of iron oxides, has been ejected at the ends of the pit. The wear scar on the corresponding Cr-steel counterbody can be seen in Fig. 11. There appear to be scratches on this scar, and no significant amounts of the ceramic could be found. The wear scars on the tungsten carbide-cobalt cemented carbide and corresponding Cr-steel counterbodies have a similar appearance to those in the case of the zirconia-toughened alumina, but the wear tracks are much smaller for the tungsten carbide-cobalt cemented carbide case.

10 pm Fig. 8. SEM photomicrographs: (a) fretting pit on the s:ialon tested against a Cr-steel counterbody for 500 000 cycles at 1 N, 100 pm and 8 Hz, (b) transfer layer and debris on the bcntom of a fretting pit on the sialon. Fig. 10. SEM photomicrograph of a fretting pit on the zirconiatoughened alumina tested against a Cr-steel counterbody for 500000 cycles at 1 N, 100 pm and 8 Hz.

Fig. 9. SEM photomicrograph of a fretting scar on a Cr -steel counterbody tested against the sialon for 500000 cycles at 1 N, 100 pm and 8 Hz.

iron and some components from the sialon. The re is compacted wear debris in and around the scar, and this debris appears to be agglomerates of smaller par-

Fig. 11. SEM photomicrograph of a fretting scar on a Cr-steel counterbody tested against the zirconia-toughened alumina for 500000 cycles at 1 N, 100 pm and 8 Hz.

P.Q. Campbell et al. f Fretting wear

54

of ceramics and cermets

Fig. 13. SEM photomicrograph of a fretting scar on a Cr-steel counterbody tested against the titanium carbonitride cermet for 500000 cycles at 1 N, 100 pm and 8 Hz.

(4

4. Discussion

(b) Fig.

in (a).

An example of a fretting pit on the titanium carbonitride cermet after wear testing for 500 000 cycles against a Cr-steel counterbody can be seen in Fig. 12(a). Relatively large scratches can be seen on the bottom of the pit, and a magn~ed~ew of these scratches can be seen in Fig. 12(b). The agglomerated debris which was ejected from the pit appears to be composed mainly of particles from the cermet. Only small traces of iron could be found in the debris, which is contrary to what was found in the other material combinations. The wear scar on the Cr-steel counterbody tested against the cermet shows sign~cant mounts of cermet particles embedded in the matrix of the softer Cr-steel, as can be seen in Fig. 13. Scratches are also evident on these embedded particles, suggesting that these particles are selectively abrading the cermet while protecting the Cr-steel.

It was mentioned in Section 2.1 that some of the fretting wear behavior might be akin to that of steel on steel, which is widely covered in the literature [2,17,19,21-231. It is postulated that the fretting wear of metals can be divided into three stages [17]: (1) initial adhesion and metal transfer, (2) production of debris in a normally oxidized state and (3) steady state wear. Berthier et al. [12] suggest that fretting can be considered as a j&w problem with its soun”es and sinks. In this case, the source is the third body consisting mainly of iron oxide debris and the sink is the area outside of the contact into which the debris is thrown. Under normal operating conditions, the steel first bodies generally produce enough oxide debris, indicating that the fretting wear rate is governed more by the removal rate of the iron oxide debris than by the formation rate of the iron oxide [12]. The exact role of the iron oxide particles depends upon the testing conditions, which determine the balance between the protective effect due to the formation of a compacted oxide layer and the abrasive action of the particles before compaction [19]. In the fretting wear of the sialon against steel, it appears that a complicated process involving both the formation of metal transfer layers and tribo-oxidation of the materials occurs. During the running-in period, contaminant films such as metal oxides are dispersed and an adhesive contact between the sialon and steel is established. As the wear process proceeds, transfer films form on both materials. Traces of iron were detected in the transfer layers on the sialon and traces of components from the sialon were found on the corresponding layers on the Cr-steel ball, indicating that these layers may be some sort of iron silicate. It has been found that at elevated temperatures (NO-900 “C) iron and manganese react chemically with the silicon

P.Q.Campbell et al. I [email protected]

nitride to form a liquid phase in air [24]. Formation of metal silicates has also been observed in the same work. The importance of such transfer films in determining the wear behavior of ceramic/metal couples has been discussed in the literature [25], where the higher wear rate of the sialon against steel, as compared to oxide ceramics, is thought to be possibly due to the oxygen in the atmosphere reacting with the silicon nitride to form silicon oxide. The silicon oxide is then easily detached from the wear surface. Also, small particles of reddish-brown iron oxide wear debris were ejected from the contact area and have formed compacted agglomerates, indicating that oxidation wear of the iron in the steel has taken place. Iron oxide debris (Fe,O,) with similar characteristics has been found in the literature [WI, and the values of the associated coefficient of friction are close to those found in this work and that of Klaffke [US]. Oxidation wear also appears to play an important role in the fretting wear of both the zir~nia-tou~ened alumina and the ~n~ten carbide-cobalt cemented carbide against Cr-steel. In both cases, the wear debris consists mainly of reddish-brown iron oxide. Once this iron oxide has formed, it abrasively wears away the Cr-steel pin and is ejected away from the contact area. The freshly exposed metal is then oxidized and the process continues. In addition, very little abrasive wear of the zirconia-toughened alumina and cemented carbide occurred, and the lower amount of abrasive wear of the cemented carbide might be attributed to its higher hardness. Also, the presence of transfer films was not detected here, but it has been found in the literature [25] that very thin dis~ntinuous films form on alumina-based materials during the reciprocating sliding of these materials against steel. It has also been found that iron oxide can firmly deposit itself on an alumina surface during pin-on-disk testing, preventing wear of the alumina disk while abrading the steel ball 1261. In the fretting wear of the titanium carbonitride cermet against the Cr-steel, a special type of abrasive wear seems to be the governing factor. It appears that the harder carbide and carbonitride particles are being debonded from the matrix of the cermet and are being embedded in the soft matrix of the C.!r-steel. These particles are then acting as fixed abrasives that are preferentially abrading the cermet while protecting the Cr-steel. It has been found in the literature [27], that a similar cermet exhibits lower friction and wear resistance than cemented carbides when they are used to machine steels. This is similar to the result that was found from the fretting testing in this work, even though the wear modes in the two cases are different. As a final note, it will be mentioned that a change in environmental factors can alter the wear behavior

wear

of ceramicsand cermets

55

of the material couples discussed here. For example, it has been found that changes in humidity can change the friction and wear behavior of alumina/steel and silicon nitride/steel couples when tested under fretting conditions [18]. Also, changes in temperature can modify the tribological behavior of these materials via promotion of tribochemical reactions, modifications in the structure of the materials, changes in the mechanical properties of the materials, etc. For example, it has been found in the literature [2.8] that the properties of tungsten carbide-cobalt cemented carbides, which exhibit the best wear resistance in this study, are altered significantly at higher temperatures. The carbides are brittle up to 500 “C, tough between 500 and 800 “C, and susceptibletoplasticdeformation above 800 “C. Thus, fretting tests performed at various temperatures and humidities might show completely different results from those obtained here.

5, Conclusions Fretting experiments with the Cr-steel counterbodies indicate that all the ceramics and cermets, with the exception of the titanium carbonitride cermet, have a steady state COF between 0.6 to 0.7, which is similar to the steady state COF found for the rubbing of steel on steel. The cermet, on the other hand, has a lower steady state COF of 0.4 to 0.5. In terms of wear volumes from room temperature wear testing, the poorest fretting wear resistance is shown by the sialon, in which case transfer layers appear to play an important role in dete~ining the fretting wear behavior. The next poorest fretting wear resistance is exhibited by the titanium carbonitride cermet, where hard particles from the cermet embed themselves in the soft Cr-steel and selectively abrade the cermet while protecting the Cr-steel. The best fretting wear resistances, on the other hand, are exhibited by the tungsten carbide-cobalt cemented carbide and the zirconiatoughened alumina, where iron oxide particles formed from the iron in the steel abrade the steel. The lower fretting wear of the tungsten carbide-cobalt cemented carbide, compared with that of the zir~nia-toughened alumina, might be attributed to the higher hardness of the carbide.

Acknowledgments The authors would like to thank C&am&al, a hard metal and ceramic manufacturer in Luxembourg, for supplying the materials, and Gtihring, a tool manufacturer in Germany, for helping to determine the relative wear performance of the materials in this work.

P.Q. Campbell et al. I Fretting wear of ceramics and cennets

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This work has been supported by the Commission of the European Community by being included within the framework of the BRITE EUFMh4 project number BREU-0096-C.

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