Sliding wear of hard materials — the importance of a fresh countermaterial surface

Wear, 124

(1988)

195 - 216

195

SLIDING WEAR OF HARD MATERIALS - THE IMPORTANCE OF A FRESH COUNTERMATERIAL SURFACE* MIKAEL OLSSON, BENGT STRIDH and STAFFAN School

of Engineering,

Uppsala University,

SeDERBERG+

Uppsala (Sweden)

ULF JANSSON Department

of Chemistry,

Uppsala University,

Uppsala (Sweden)

summary The possibility of reproducing the contact conditions at a tool-workpiece interface in machining, using a laboratory wear test, has been investigated. For this purpose, a modified pin-on-ring test with continuous introduction of fresh countermaterial was designed. Using this equipment the wear characteristics of several hard materials (high speed steel, uncoated and coated cemented carbide), sliding against a quenched and tempered steel, were investigated. The results obtained in the modified test were compared with results from conventional pin-on-ring testing, and machining. Post-test metallographic examination demonstrated that it is possible to reproduce the contact conditions and wear processes in machining under controlled laboratory conditions using the modified pin-on-ring test. The dominant sliding wear mechanisms for hard materials against a fresh surface appear to be adhesive wear and solution wear. A conventional wear test cannot be used for wear testing of tool materials since transfer of pm material, depletion of alloying elements and changes in contact geometry produce a contact condition which differs significantly from the intended application.

1. Introduction Wear of materials in continuous sliding contact can either occur by cyclic reintroduction of the same surface elements from the countermaterial or by continuous introduction of new material to the interface. In engineering applications both types of contact conditions are found. Repeated contact occurs between many machine elements, such as journal bearings, rotating seals and piston rings. By contrast, industrial tools generally slide against a fresh, not previously encountered, countermaterial surface. Examples of *Paper presented at the International Conference on Wear of Materials, Houston, TX, U.S.A., April 6 - 9,1987. +Present address: Sandvik Coromant AB, Box 42056, S-12612 Stockholm, Sweden. 0043-1648/88/$3.50

0 Elsevier Sequoia/Printed

in The Netherlands

196

this latter category include cutting and forming tools, drawing dies and scraper blades. In laboratory wear testing it is important to reproduce the contact conditions of the intended application [ 1, 21. The standard laboratory test set-ups for sliding wear are either of pin-on-disc or pin-on-ring type, both characterized by repeated contact between the surfaces. This design has obvious experimental advantages but the resulting contact conditions differ from those at a tool-workpiece interface. Thus most attempts to use conventional testing for prediction of the wear resistance of tool materials have been unsuccessful [3, 41. Instead, wear testing of cutting tool materials are usually performed by machining tests [ 5 - lo]. In the present paper, the effect of a fresh countermaterial surface on the wear behaviour of high speed steel and uncoated, Tic-coated and B,CTic-coated cemented carbide sliding against a quenched and tempered steel has been investigated. In order to achieve this type of test condition, a modified pinon-ring test set-up has been designed. The resulting wear characteristics have been compared with those observed in conventional pin-on-ring testing, and machining, the principle goal being to explore the possibilities of simulating tool wear mechanisms in machining using a laboratory wear test. There are three dominant mechanisms of tool wear that are widely accepted in the literature. (1) Mild adhesive wear due to the formation of adhesive junctions at the tool-workpiece interface. This results in superficial shear deformation of the tool material in very thin layers. A wear particle is detached when the shear fracture strain of the tool material is exceeded [lo, 111. (2) Solution wear by atomic transport of tool material to the workpiece [5, 6, 121. The detailed mechanism of this wear process is still poorly known and some authors prefer the term diffusion wear [ 131. (3) Abrasive wear due to the grooving action of hard second-phase particles or inclusions in the workpiece material. Tool material can be removed by microspalling around the groove or, more commonly, by chip formation [14,15]. There are additional mechanisms involved in the deterioration of the cutting edge in machining, such as edge blunting by chipping or plastic deformation [8]. This poses a problem in the interpretation of data from comparative tool material testing since a poor performance may be due to either insufficient toughness, strength or wear resistance. A laboratory wear test that simulates tool wear will allow the wear resistance to be determined separately.

2. Experimental details 2.1. Test equipment A pin-on-ring test set-up with continuous introduction of fresh countermaterial was accomplished by mounting the ring material as workpiece in a

197

Fig. 1. Schematic illustration of the modified pin-on-ring test in which the pin A is made to slide continuously against fresh countermaterial surface B.

lathe. The pin is mounted on the tool holder and a spring loading system is used to control the applied normal force. During the test, the frictional force is monitored using strain gauges. Wear testing against fresh material can now be performed using the feed control of the lathe, see Fig. 1. Before the test, a fine-cutting tool is used to produce a fresh surface on the cylindrical workpiece. In the present experiments a workpiece (0.9 m long) of diameter 0.14 m was used as ring material. This aIlowed a maximum of about 1000 m sliding distance before a new surface has to be produced. Conventional pinon-ring testing can be performed using the same equipment by disengaging the feed control. Further details of the experimental set-up have been presented earlier [ 161. 2.2. Materials Wear tests were performed with five pin materials: high speed steel and uncoated, Tic-coated, B,C-TiCcoated and TiCcoated cemented carbide, the latter heat treated under exactly the same conditions as during B,C deposition. In Table 1 chemical compositions and hardness values of the high speed steel and cemented carbide are given. The TiC coating was deposited by chemical vapour deposition using a Sandvik Coromant standard procedure [17]. The B,C coating was also deposited by chemical vapour deposiTABLE 1 Chemical composition and hardness values for pin and ring materials Specimen

Designation

Chemical composition

(wt.%)

Vickers’ hardness (200 N load) (HV)

Pin

ASP 30a

1.30 C, 0.5 Si, 0.3 Mn, 4.0 Cr, 4.8 MO, 6.1 W, 3.1 V, 8.0 Co

1030 f 30

Pin

H 20b

92.0 WC, 2.0 (Ta,Nb)C, 6.0 Co

1450 * 20

Ring

AISI 4340

0.35 C, 0.3 Si, 0.6 Mn, 1.4 Cr, 3.3 Ni, 0.3 MO

300 f 15

aKloster Speedsteel designation. bSandvik Coromant designation.

198

tion, using a low pressure system with a hot wall quartz reactor (described elsewhere [ 181). Deposition of boron carbide was obtained using a gas mixture of 8.2 vol.% BCl, and 9.8 vol.% CH4 with H, as a carrier gas. A deposition temperature of 1030 ‘Z, total gas pressure of 6.7 kPa and deposition time of 17 min were used. The B,C! coating had the composition B4.,C, according to Auger analysis, X-ray diffraction showed the B,C structure to be amorphous with a thin layer of TiB2 at the Tic-B,C interface. The coating thicknesses of the TiC and B,C were measured to be 3.5 - 3.9 pm and 1.6 - 2 .O pm respectively. In all tests a quenched and tempered, low alloy steel (AISI 4340) was used as ring material. This steel is frequently used as a workpiece in tool life testing of metal cutting tools. The chemical composition and hardness value of the AISI 4340 steel are also given in Table 1. 2.3. Test procedure Wear tests were performed, for all five pin materials, both with and without introduction of fresh countermaterial. A normal force of 500 N was applied in all tests. The sliding speed was 75 m min-’ for the high speed steel and 250 m mm-’ for the other four pin materials. Tests were performed corresponding to 10, 25, 50, 100 and 300 m sliding distance and a minimum of two tests were performed for each sliding distance and test mode. Machining tests were performed with tools made from the five pin materials using the same workpiece material as in the wear tests. Twist drilling was selected as a typical application for the high speed steel while turning tests were performed for the coated and uncoated cemented carbides. The cutting speeds used in the drilling and turning tests were 45 m min-’ and 190 m mini respectively. 2.4. Analysis The worn pins and cutting tools were subjected to a careful metallographic characterization using light optical microscopy (LOM), scanning electron microscopy (SEM) , energydispersive X-ray spectroscopy (EDX), wavelengthdispersive X-ray spectroscopy (WDX) and Auger electron spectroscopy (AES). Polished and etched cross-sections were analysed using LOM. One problem in wear testing of hard materials under realistic testing conditions is that the wear rates are generally very low and weight loss measurements cannot be utilized, especially since extensive material transfer from the softer to the harder material often occurs [ 191. In tool life testing of cutting tools, wear is usually represented by the size of the wear scar [20]. This method was also employed in the present experiments. However, misleading results may be obtained since, for low volume losses, the measured wear scar area will yield a value that is more representative of the total contact area than of the actual volume of material removed. In the case of the coated pins, this problem was circumvented by measuring only the area of exposed cemented carbide using the compositional mode in SEM, see

(a)

(b)

Fig. 2. Micrographs of a wear scar produced on a B,C-Tic-coated cemented carbide pin: (a) SEM, secondary electron mode; (b) SEM, compositional mode. The white areas in the centre of the scar correspond to areas where the coating has been removed and the underlying cemented carbide is exposed.

Fig. 2. For all cemented carbide pins, it was necessary to first use an etching treatment (40% HCl at boiling temperature) to remove adhered work material from the wear scar. 3. Results 3.1. Wear scar area and geometry The results of the wear tests are summarized in Fig. 3, in which the wear scar area or, in the case of coated cemented carbide, the area of exposed substrate material, is plotted us. sliding distance. In addition, the geometry and gradual growth of the wear scar are illustrated by schematic wear scar maps. The heat-treated Tic-coated pins are not included in the figure since they display an identical behaviour to the non-heat-treated pins. The wear scar maps in the figure demonstrate that the geometry of the wear scars differs depending on the pin material and test mode. In the modified test an elliptical wear scar is obtained with a geometry that almost matches the ideal shape corresponding to the crossed cylinders geometry. As long as this is the case, it is possible to convert the measured wear scar areas to wear volumes, using the equations given by Halling [21]. If this is done, it is found that the logarithmic curve shapes in Fig. 3 correspond to a linear increase in wear volume with sliding distance, i.e. a constant wear rate (wear volume per unit sliding distance). By contrast, the wear scars have a more rectangular shape in the conventional test. This deviation from the ideal geometry is explained by the fact that a groove with ridges on both sides is produced when the pin is sliding repeatedly in the same wear track. The change in contact geometry with time results in a shallow wear scar on the pins, elongated perpendicular to the sliding direction. Consequently, the wear will be overestimated in the conventional test mode if the wear scar area is taken as a measure of wear.

h d Fig. 3. (a - d) Wear scar area vs. sliding distance for conventional (open circles) and modified (closed circles) pin-on-ring tests. For the coated pins the area refers to the exposed cemented carbide area. (e - h) Examples of wear scars (after etching) as a function of sliding distance and test mode. Worn TiC or B,C, exposed cemented carbide and adhered glassy layers are indicated by grey, white and black respectively in (f - h). Pin materials: high speed steel (a, e) and uncoated (b, f), Tic-coated (c, g) and B,C-TIC coated (d, h) cemented carbide.

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For the coated pins this problem was avoided by using the area of exposed substrate material as a measure of wear. For the high speed steel pins the total wear scar area could be used with little error since the wear scars on this material showed little deviation from the ideal elliptical shape. In the case of the uncoated cemented carbide pins the larger wear scar area produced in the conventional test as compared with the modified test does not necessarily imply a higher wear rate. A comparison between the wear scar maps of uncoated and coated pins suggests that all cemented carbide pin materials display the same wear behaviour, i.e. a significantly higher wear rate in the modified test, as compared with conventional testing. By contrast, the conventional test yields the higher wear rate for the high speed steel pins. The wear scar maps also show that the pins are still partly covered by adhered material after the etching treatment. This also applies to the high speed steel pins but, in this case, the individual area elements of adhered material are too small to be indicated in the maps. 3.2. Coefficient of friction The recorded coefficients of friction and test modes are given in Table 2. In all is observed for the modified test mode testing. The friction coefficient is lower for cemented carbide pins.

p for the different pin materials cases a slightly higher value of ~1 as compared with conventional the coated than for the uncoated

3.3. Topography and microstructure of worn surfaces 3.3.1. High speed steel The conventional pin-on-ring test yields a rough wear scar with a large number of scales, see Fig. 4(a). LOM cross-sections reveal superficial plastic flow of the pin material with whiteetching, flake-like high speed steel psrtitles on the surface, see Fig. 4(b). These particles appear to have been transTABLE 2 Friction coefficient values /J recorded in wear experiments. It should be noted that the sliding speed was 75 m min-’ for the high speed steel and 250 m min-’ for the other materials Material

Test mode

P

High speed steel High speed steel

Conventional pin on ring Modified pin on ring

0.50 0.55

Uncoated WC-Co Uncoated WC-Co

Conventional pin on ring Modified pin on ring

0.35 0.45

Tic-coated Tic-coated

Conventional pin on ring Modified pin on ring

0.30 0.35

Conventional pin on ring Modified pin on ring

0.30 0.35

WC-Co WC-Co

B,C-Tic-coated B,C-TiCcoated

WC-Co WC-Co

(a)

(b)

(c) Fig. 4. Characteristic features of worn high speed steel pins from a conventional test, sliding distance 300 m. (a) Rough surface with scales. (b) Etched cross-section with whiteetching high speed steel transfer layer (at A) and superficial plastic flow (at B). (c) Adhered fragment of high speed steel (at C) on the corresponding ring surface.

ferred back and forth between the pin and ring surfaces during sliding. This is supported by the observation of a large number of adhered and heavily deformed, whiteetching high speed steel particles on the corresponding ring surface, see Fig. 4(c). The colours produced by etching show that temperatures well above 600 “C have been generated in the superficial layer of the pin [22]. Sliding against a fresh countermaterial surface produces a wear scar that is characterized by a large number of shallow craters, see Fig. 5(a). No signs of a plastically deformed surface layer can be seen on etched cross-sections and the superficial temperature was lower than in the conventional mode, see Fig. 5(b). Small areas with thin, adherent layers with high contents of silicon, manganese and chromium are found on the wear scar, as indicated in Fig. 5(b). In the scanning electron microscope these layers display a “glassy” morphology. 3.3.2. Uncoated cemented carbide The surface of the worn pins from the conventional test is to a large extent concealed by layers of adhered, glassy material (1 - 2 pm thick) that extend over large portions of the scar, see Fig. 6(a). At the points where the

(4

(b)

Fig. 5. Characteristic features of worn high speed steel pins from the modified wear test, sliding distance 300 m. (a) Shallow crater (at A). (b) Etched cross-section with shallow craters and adhered glassy layer (at B).

(cl Fig. 6. Characteristic features of worn uncoated cemented carbide pins from the conventional wear test. (a) Adhered glassy layer (at A) after 25 m sliding distance. (b) Extremely smooth wear scar topography with shallow grooves after 300 m sliding distance. (c) Ring surface with WC particles from the pin in (b).

pin surface is exposed, an extremely smooth wear scar topography with shallow grooves is seen, cfi Fig. 6(b). The topography resembles that of a mechanically polished surface. The surface porosity that is seen in Fig. 6(b) is caused by the fact that carbide particles are torn out of the cemented carbide during the test. (After etching, an additional contribution to the

204

porosity is obtained since the etchant attacks the cobalt binder phase.) Using the compositional mode in SEM and EDX, WC particles can be found on the ring surface, see Fig. 6(c). Between 100 and 300 m sliding distance, the number of carbide particles torn out of the surface increases drastically. This is probably related to the fact that the adhered, glassy layers are found to disappear completely at a sliding distance greater than 100 m. The resulting change in contact conditions produces a slight increase in the coefficient of friction. The modified mode also produces a smooth worn surface, the difference being that, in this case, protruding carbide particles are observed. These were identified by EDX as complex carbides of the (Ta,Nb)C type, see Fig. 7(a). Larger grooves are seen towards the exit side of the wear scar, see Fig. 7(b). Adhered glassy layers are formed also in this test mode and were still found on the surface after a sliding distance of 300 m.

(a)

(b)

Fig. 7. Characteristic features of uncoated cemented carbide pins from the modified wear test, sliding distance 100 m. (a) Smooth wear scar with protruding (Ta,Nb)C grains. (b) Abrasive grooves at the exit side of the scar.

3.3.3. Tic-coated cemented carbide After a conventional test, the worn pins are to a large extent covered by adhered, glassy layers. At the few points where the TiC surface is exposed, the coating appears to be virtually unaffected with a topography resembling that of the asdeposited coating, see Fig. S(a). However, a few small pits where the coating has worn through and the cemented carbide is exposed are observed after 300 m, see Fig. 8(b). In contrast to the conventional test with uncoated pins, the adhered glassy layers are still present after the longest sliding distance. In the modified test mode, fine scales develop on the TiC coating already after 10 m sliding distance, see Fig. 9(a). The thickness of the coating is gradually reduced by wear and, eventually, the underlying cemented carbide is exposed. When this occurs a deep crater is rapidly formed owing to the significantly higher wear rate of the cemented carbide, see Fig. 9(b). The topography in the crater is similar to that of the uncoated pins at the same experimental conditions. The craters are often surrounded by glassy layers.

(4

(b)

Fig. 8. Characteristic features of Tic-coated cemented carbide pins from the conventional wear test. (a) Adhered glassy layers (at A) with no signs of coating wear after 100 m sliding distance. (b) Pit in TiC coating with exposed cemented carbide (at B) after 300 m sliding distance.

(cl Fig. 9. Characteristic features of Tic-coated cemented carbide pins from the modified wear test. (a) Fine surface scales on the TIC coating after 10 m sliding distance. (b) Crater formation (at A) with exposed cemented carbide after 300 m sliding distance. (c) Transverse crack across the wear scar.

An additional feature of the Tic-coated pins in the modified test is the formation of transverse cracks across the wear scar, see Fig. 9(c). The cracks appear to have been nucleated at the centre of the scar and propagate perpendicular to the direction of sliding. Not only do the cracks propagate through the TiC coating and the underlying substrate material, but also through the glassy layers. In addition, a network of smaller cracks is

206

observed, formed already during specimen cooling from the deposition temperature. In spite of these cracks, the TiC coating adheres well to the substrate. Spalling, the size of which agrees with the dimensions of the crack network, is only sporadically observed on the extreme entrance side of the scar. The heat-treated Tic-coated pins displayed identical wear characteristics compared with the non-heat-treated pins. 3.3.4. B,C-Tic-coated cemented carbide The wear scars of the B,C-Tic-coated pins display many of the characteristics of the Tic-coated pins. The B,C! coating itself appears to wear very rapidly. The worn B,C is characterized by an extremely smooth topography which develops after short sliding distances in both test modes, see Fig. 10(a). When the B,C coating is worn through, wear proceeds, for both test modes, by the same wear mechanisms as previously described for the TiC coating and cemented carbide. The main difference between B,C-TiC- and Tic-coated pins is that the former display a pronounced propensity to spalling damage. The spalling depth varies. Interfacial spalling along the B,C-TiC or, more commonly,

(a)

(b)

(cl Fig. 10. Characteristic features of B,C-Tic-coated cemented carbide pins from the modified wear test, after 300 m sliding distance. (a) Extremely smooth wear topography of B,C (at A) with unaffected TiC (at B) where the B,C has worn through. The as-deposited topography of B,C is seen at the top (at C). (b) Large spalling at the entrance side of the wear scar. (c) Gouging wear marks toward the exit side of the scar.

along the Tic-cemented carbide interface is not only observed but also spalling that penetrates into the cemented carbide substrate, see Fig. 10(b). Large spalled areas are found at the back of the wear scar, see Fig. 10(c), while smaller spalling is found at the extreme entrance side. The spalling causes the underlying cemented carbide to be exposed and, consequently, the wear of the B,C-Tic-coated pins becomes greater than for the TiCcoated pins. This effect is especially noticeable in the conventional test where the TiCcoated pins display almost negligible wear. 3.4. Morphology and composition of adhered glassy layers 3.4.1. Morphology Adhered glassy layers were observed on all pin materials in the present tests, although only small areas with very thin layers were observed on the high speed steel and only in the modified test mode. In Fig. 11 an example of an adhered glassy layer is seen in high magnification. Before etching, the glassy layers are often difficult to identify since the pins are covered with thick layers of adhered ring material. SEM examination reveals that the glassy layers are non-metallic since charging occurs if the pins are not coated with a conducting material. The smooth surface and smeared appearance of the layers indicate that they have been plastically deformed during the test. The layers appear to adhere well to the pm surface and are probably gradually built-up during sliding until they become mechanically unstable and large fragments are removed. Large areas with glassy layers are most commonly observed at the back of the wear scar and around the deep craters formed on the coated pins.

Fig. 11. Adhered glassy layer on Tic-coated cemented carbide, after 300 m sliding distance. The inset shows the corresponding EDX spectrum.

3.4.2. Composition of glassy layers EDX analysis reveals that the glassy layers mainly contain aluminium, silicon, manganese, chromium, iron and, in the case of Tic-coated pins, titanium. Using AES and WDX oxygen and carbon were also detected. Very large variations in the chemical composition of the layers are observed on the same wear scar, see the EDX results in Fig. 12. In Fig. 12, oxygen and carbon are excluded because it was not possible to detect these elements using the

.

1

PERCENT

Fig. 12. Illustration modified test. The positions.

50 0

I

Is

WIGHT

3 4

at.%) of aluminium,

on a wear scar on uncoated cemented carbide from the silicon, chromium, manganese and iron at the indicated

7

LAYERS

of the glassy layers

OF GLASSY

of the variation in chemical composition diagrams show the relative amounts (in

2

EDS ANALYSIS

t.2 z2

209

EDX equipment available. A rough estimation from AES and WDX gives an oxygen content and carbon content of approximately 40 at.% and 10 at.% respectively. It is difficult to present quantitative data on the chemical composition of the layers because of charging during the analysis and the pressence of light elements such as oxygen and carbon. Charging of the layers was a problem especially in Auger analysis and we were unable to acquire Auger depth profiles. 3.5. Wear characteristics of machining tools In this section the wear characteristics of the pin materials, when used as cutting tools, will be described. Since all the wear characteristics of uncoated and Tic-coated cemented carbide can also be found on the B&J-Ticcoated tools, the presentation is restricted to high speed steel and B,C-TiCcoated cemented carbide. 3.5.1. High speed steel cutting tools The high speed steel twist drills are mainly worn by flank wear, i.e. the gradual flattening out of the cutting edge on the clearance face of the tool. The flank is to a large extent covered with adhered work material but careful examination in the scanning electron microscope reveals that shallow craters are formed on the worn surface at a short distance behind the cutting edge, see Fig. 13(a). The etched cross-section in Fig. 13(b) shows that no deformation or thermal effects are resolved below the flank. These characteristics are very similar to what was earlier described for the modified wear test with a fresh countermaterial surface, cf. Fig. 5.

(a)

(b)

Fig. 13. Characteristic features of a worn high speed steel twist drill. (a) Shallow craters on drill flank (at A). (b) Etched cross-section of a shallow crater showing unaffected high speed steel material under the flank.

3.5.2. B,C-Tic-coated cemented carbide tools The B,C-TiCcoated tools display both flank and crater wear, the latter generally causing the most severe wear damage. The B,C coating is worn rapidly and can only be found near the cutting edge on worn tools. Figure 14(a) illustrates that worn B,C exhibits an extremely smooth surface which closely corresponds to what was observed around the periphery of the

(a)

(b)

Cc) Fig. 14. Characteristic features of crater wear on B,C-Tic-coated cemented carbide inserts from turning tests. (a) Smooth topography of B,C near the cutting edge (at A). (b) Surface scales on worn Tic. (c) Smooth topography of exposed cemented carbide with protruding (Ta,Nb)C grains.

wear scars in both the conventional and the modified test. In the crater the TiC coating is exposed and the same type of surface scales as in the modified test are observed, see Fig. 14(b). If the cutting time is increased, the TiC coating is worn through and the cemented carbide is exposed. Figure 14(c) illustrates that the worn cemented carbide exhibits a smooth topography with protruding (Ta,Nb)C particles. This is also very similar to the observed features of the modified wear test. The only major difference between the worn cutting tools and the pins from the modified wear test is that adhered glassy layers were not found on the tools. 4. Discussion 4.1. Glassy layer formation The composition of the glassy layers indicates that they are formed by a chemical reaction between different alloying elements from the ring material. Silicon and aluminium are added to the AISI 4340 steel as oxide-forming elements while manganese forms sulphides. The existence of oxide inclusions in the steel implies that oxygen from the ambient atmosphere is not required for the chemical reaction to take place. Examination of the literature shows

211

that a large number of chemical compounds can be formed between the chemical elements observed in the layers (principally silicon, manganese, chromium and oxygen) and that there exist several eutectic compositions for which melting occurs already at 1200 - 1300 “C [23]. Measurements of the maximum temperatures generated in metal cutting tools at cutting speeds corresponding to the sliding speed in the present experiments show that temperatures well above 1000 “C can be obtained in the tool [24]. The fact that nickel and sulphur are never detected in the glassy layers shows that not all of the chemical elements in the ring material are involved in glassy layer formation. The disappearance of the glassy layers from the surface of the uncoated cemented carbide pins after long sliding distances in the conventional test, is interpreted as an effect of repeated contact in the same wear track. If larger fragments of the glassy layer are removed during sliding, the layer may be able to reform a few times but, eventually, the ring surface will become depleted of the glass-forming elements and no further layer formation can occur. There is no tendency of the layers to disappear in the modified test since new ring material is constantly brought into contact with the pin. The area fraction of the total wear scar that is covered by glassy layers is generally somewhat larger for the coated than for the uncoated pins. This suggests that TiC and B,C are more favourable than WC as substrate material in glassy layer formation. EDX analysis revealed significant amounts of titanium in the glassy layers on the coated pins. Thus it appears that titanium (and probably also carbon) is dissolved into the glassy layers. This results in a less well-defined interface and probably an improved adhesion between layer and coating. This may be favourable for wear resistance since the glassy layers appear to protect the underlying pin material. 4.2. Relevance of laboratory testing of cutting tool materials

4.2.1. Modified pin-on-ring test The metallographic examination of the worn pins and cutting tools from the present experiments has shown that the modified test with continuous introduction of a fresh countermaterial surface is able to reproduce closely the wear characteristics of machining tools. On the high speed steel a characteristic wear topography with a large number of shallow craters is formed. Earlier work [8, 111 has shown that the craters develop as a result of mild adhesive wear caused by the repeated shearing of asperity contact junctions at the interface. The high speed steel is only sheared in a very thin surface layer which is not possible to resolve using LOM. The formation of a smooth surface with protruding complex carbide grains on the worn cemented carbide is well known from machining tests [6, 91. This topography is often considered as evidence for solution wear of the cemented carbide, i.e. wear due to dissolution of the tungsten and carbon into the counter-material [25]. The fact that the complex carbides protrude from the surface can be explained as an effect of their superior chemical stability as compared with WC. Addition of complex carbides is

212

also known to enhance the wear resistance of cemented carbides at cutting conditions where solution wear can be expected [6]. However, attempts to verify the solution wear mechanism were unsuccessful since no elements from the pin could be detected on the ring surface using Auger depth profiling. The abrasive grooves found at the rear of the wear scars are probably caused by the complex carbide grains when they are, eventually, torn out of the cemented carbide. The formation of small, thin scales on the surface of the TiC coating indicates wear by localized plastic shearing at the surface, followed by fracture and detachment of the scales. This wear mechanism has been described earlier for Tic-coated machining tools [lo]. It is a relatively mild wear process and the thickness of the coating is only gradually reduced until the underlying cemented carbide substrate is exposed. As soon as this happens, rapid wear of the cemented carbide occurs by solution wear and a crater is formed. The significantly higher wear rate of the cemented carbide as compared with TiC results in the formation of an overhang of TiC coating around the crater. Eventually, this overhang will collapse and the crater continues to grow in size. If a glassy layer is present, the continued growth of the crater is slowed down owing to the protective effect of these layers. Consequently, after long sliding distances, the wear scars display a characteristic appearance with one or several large craters surrounded by protective layers of adhered, glassy material. The mechanism behind the formation of the extremely smooth surface topography on the worn B,C coatings is not clear. In the case of uncoated cemented carbide, a smooth topography is often considered to be a strong indication of a solution wear mechanism. A smooth surface could also be produced by mechanical polishing but, if this was the case, B,C would be expected to wear less than TIC because of its greater hardness. An alternative mechanism would be a two-step process with transformation of B,C by an interfacial chemical reaction followed by mechanical removal of the reaction layer. An attempt to predict the solution wear resistance of B,C was made using the theoretical model proposed by Kramer and Suh [26]. The wear resistance of B4C was computed to be about four times less than WC, a result which is in qualitative agreement with our experimental findings. SEM examination reveals that, once the B,C coating has worn through, the asdeposited topography of the underlying TiC coating will reappear, cfi Fig. 10(a). This clearly demonstrates the superior wear resistance of TiC compared with B,C under the present experimental conditions. An additional feature of the B,C-Tic-coated pins is that surface damage in the form of spalling or chipping is observed. Spalling damage by delamination of surface layers occurs on the entrance side of the wear scar. The spalling often extends into the cemented carbide substrate, probably caused by the embrittling effect of eta-phase formation during TiC deposition [ 271. A high density of small chippings is observed further back on the wear scar, c/‘. Fig. 10(c). Their geometry suggests that they have been produced by some kind of gouging action. The spalled layers from the entrance

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side of the pin may cause the observed damage as they are being transported backwards between the contacting surfaces. The fact that spalling and chipping damage are observed for B&-TICcoated pins but not for TiCcoated pins is somewhat surprising. There are no indications of the spalling being caused by poor adhesion at the B,C-TiC interface and the heat-treated TiCcoated pins do not display spalling along the Tic-cemented carbide interface. Instead, it appears that the spalling is caused by a reduction in the fracture toughness of the surface material. This may be an effect of the higher hardness of the B,C as compared with Tic. The larger total coating thickness will also contribute to the decrease in fracture toughness [ 281. There are no indications that the large transverse cracks that form on the coated cemented carbide pins play a role in the continuous wear of the pins. However, in a practical application, crack formation would generally be detrimental since most engineering components are subjected to additional dynamic loading. The only major discrepancy between the wear characteristics found in the modified test as compared with machining is the glassy layer formation which was only observed in the laboratory test. However, glassy layer formation has been observed in other studies on machining tools [29, 301. It has been found that glassy layer formation is strongly dependent on the steelmaking practice employed. Even different batches of the same material from the same supplier often give completely different results [31]. This may explain the difference between the laboratory and cutting tests since batches from two different suppliers were used. In addition, the sliding speeds used in the laboratory tests were higher than in the cutting tests. Therefore it is possible that the temperatures at the tool-workpiece interface were lower than at the pin-ring interface. 4.2.2. Conventional pin-on-ring test The results from the conventional tests clearly demonstrate the hazards in trying to apply standard wear tests to tool applications. It is interesting to note that a conventional test may give a value of the wear rate that is too high (as for high speed steel) or too low (as for coated cemented carbide). For the high speed steel pins, severe adhesive wear occurs owing to the transfer of pin material to the ring during the test. This creates a contact situation where high speed steel is sliding against high speed steel. Wear will predominantly occur on the pin since each individual surface element on the ring only experiences intermittent contact and, consequently, will maintain a lower surface temperature. Furthermore, heat conduction is more efficient in the ring because of its larger volume. This implies that the high speed steel particles on the ring will be harder and more wear resistant than the pin material. For the uncoated and coated cemented carbide pins, transfer of pin material is observed to a very limited extent. This is, at least partly, explained by the formation of stable glassy layers. It is only when the glassy layers dis-

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appear from the uncoated pins above 100 m sliding distance that significant transfer is observed. It may also be noted that the solution wear mechanism can never be reproduced by a test with repeated contact since the build-up of a concentration gradient in the countermaterial will arrest further dissolution. The TiC coating displays very little wear in the conventional test, possibly because of the fact that the glassy layers adhere extremely well to the TiC coating and protect it from direct contact with the workpiece. By contrast, the B,C coating exhibits the same type of smooth topography as in the modified test. This indicates that the B,C coating is extremely chemically unstable under the prevailing conditions at the sliding interface and solution wear or mechanical removal of reaction layers will occur in spite of the less severe contact conditions as compared with the modified test. 5. Conclusions The present work demonstrates that it is possible to reproduce closely the contact conditions in machining by using a modified pin-on-ring test in which the pm is made to slide continuously against a fresh countermaterial surface. Using a quenched and tempered steel as countermaterial, it was found that the high speed steel and TiC wear predominantly by mild adhesive wear, while the wear characteristics of cemented carbide and B,C suggest solution wear, although no definite support for this mechanism could be detected by Auger depth profiling. Abrasive wear is observed as a concurrent wear mechanism for all tested pin materials, although its contribution to the total wear rate is small in the present experiments. This is in agreement with the fact that abrasive wear in machining is mainly observed at lower cutting speeds [8]. Of particular interest for future research on tool materials is the ability to produce contact conditions generally associated with solution wear in a laboratory test. If the results from the conventional pin-on-ring tests are compared with the results from the modified test, several important differences are observed. The following observations are true for the conventional test but not for the modified test. (1) Transfer of wear particles from the pin to the ring surface can occur. In the extreme case this leads to a contact situation where the pin material is sliding against itself which affects both wear rates and wear mechanisms. (2) For conditions corresponding to solution wear, repeated contact between the same surface elements will result in a gradual build-up of a concentration gradient across the interface until the dissolution process is completely arrested. (3) Abrasive wear caused by hard particles in the countermaterial will not occur except during a short running-in period since the abrasive elements will rapidly wear and lose their abrasive ability. (4) The formation of interfaciai layers is also affected since the ring material will become gradually depleted of the alloying elements that react to form the glassy layer.

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(5) Groove formation on the ring surface results in a contact geometry that is poorly controlled and time dependent. Together, these characteristics produce a contact situation that is completely different from sliding against a fresh surface. Conventional pin-onring testing can, therefore, not be applied in research on wear of tool materials. By contrast, the ability of the modified test to reproduce wear mechanisms in machining under controlled laboratory conditions appears very promising and further studies on the wear of hard materials using this equipment are expected to prove fruitful.

Acknowledgments The authors wish to express their gratitude to Professor Olof Vingsbo and Dr. Jan-Otto Carlsson at Uppsala University, and Klas-Goran Stjernberg and Anders Thelin at Sandvik Coromant, for their support and helpful discussion of this work. We also wish to thank Claes Blinder, SGU, for the WDX analysis. The financial support of the National Swedish Board for Technical Development is acknowledged.

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