Examining The Definition Of Ceramic Metal Composites Construction Essay

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Application of carbide composites (hardmetals, cermets) enables service life of tools and wearing machine parts to be prolonged. These materials are mainly used in service conditions where high wear resistance either in abrasive conditions or at elevated temperatures (high speed cutting) is required. Carbide composites are not so often used in non-cutting (metalforming) operations owing to complicated service conditions where severe wear (particularly adhesive wear) is accompanied by high mechanical loads of cyclic and dynamic nature. In such conditions primarily special cold or hot working steels or hardmetals with increased binder content have been successfully used 1, 5.

Tallinn University of Technology (TUT) has developed a series of TiC-based cermets, particularly tool materials for plastic forming of metals. One of the most successful cermets  grade T70/14 (70 wt% TiC cemented with Ni-steel)  has proven itself as a tool material in blanking of sheet metals 6, 7.

The present paper is focused on the study of performance in cyclic loading wear conditions of an advanced (with improved properties) cermet  grade TiC75/14-H produced by optimized technology (Sinter/HIP, heat treatment) 8, 9. As reference materials, WC-based hardmetal grade C13 (87 wt% WC) widely used in metalforming and the ordinary TiC-based cermet grade T70/14 (70 wt% TiC) were also under investigation.

Experimental

2.1. Materials

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The main microstructural characteristics and mechanical properties of tested carbide composites are presented in Fig.1 and Table 1. Fig.1 shows the microstructures of the composites with carbide fraction of 80 vol. %. The microstructure of WC-composite (grade C13) consists of WC-grains mainly of angular shape embedded in the binder phase. The shape of TiC grains is more rounded.

B

A

Fig. 1 SEM images of investigated carbide composites

A - WC-based hardmetal (C13), B - TiC-based cermet (T75/14-H)

The alloys were sintered by two techniques: ordinary vacuum sintering (hardmetal grade C13 and cermet T 70/14) and by combined sintering in vacuum + argon gas compression (cermet grade T75/14-H) - developed for TiC-based cermet sinter/HIP technology 8, 9.

Table 1. Structural characteristics and properties (hardness HV, transverse rupture strength RTZ, proof stress RCO.1) of carbide composites tested

Grade

Carbide,

wt%

Mean grain size,

m

Binder

HV,

GPa

RTZ,

GPa

RCO.1,

GPa

C13

T70/14

T75/14-H

WC, 87

TiC, 70

TiC, 75

1.8

2.1

2.0

Co (W)

Fe (14Ni)

Fe (14Ni)

13.0

12.8

13.6

2.9

2.2

2.4

2.8

2.4

2.8

Testing procedures

Durability (blanking performance) trials were carried out as functional ones in blanking on an automatic press of grooves into electrotechnical sheet steel (see Fig.2) by a 3-position die reinforced with alloys presented in Table 1 6, 7. Durability was evaluated by the side wear of the die  (increase in diameter) after an intermediate service time N=0.5 ïƒ- 106 strokes (as N/D) corresponding to the time between two consecutive prophylactic sharpenings (used in the exploitation of blanking dies) 7. The side wear was measured using the measuring machine STRATO 9-166 in fixed environmental conditions (constant room temperature of 20  2ï‚°C and relative humidity of 50...60%) as an average of five measurements.

Fig.2 Durability testing of carbide composites in 3-position blanking die, mounted on

the automatic press

The wear was studied in cutting adhesive wear conditions 7. The wear resistance L1 was determined as the cutting path (by turning mild steel) when the track at the nose of a specimen (tool) exceeded 1 mm.

Fatigue tests resembled those of three point bending fatigue  fatigue of specimen of 5 x 5 x 17 mm under sinusoidally alternating transverse bending load at the stress ratio R = 0.1 and frequency f = 30 Hz 10 - 13. The resistance to fatigue damage was characterized by the factor of fatigue sensitivity intensity in the decrease of strength with an increase in the loading cycles from N3 = 103 to N7 = 107 as S 3-7.

Examinations were complemented by SEM and XRD studies of micro- and fine structure, performed on the electron microscope JEOL JSM 840A and diffractometer Bruker D5005, respectively. Changes of line intensities of X-ray reflections from composites phases (line peaks) and their broadening (both characterizing changes in fine structure) were determined in the XRD studies 14, 15.

Results and discussion

Fig. 3 Wear contours of blanking tools (their cutting edges)

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A - side wear D1 of dies, B - side wear D2 of punches

Results of functional wear tests are shown in Fig. 3 as wear contours H (side wear  depending on the depth H from the cutting edge of the tool). The wear contours both of the die and punch refer to a superiority of the advanced TiC-based cermet T75/14-H over the hardmetal and cermet T70/14. The blanking performance N/D (N = 0.5ïƒ-106 strokes) of the advanced composite exceeds that of ordinary composites by a factor of 1.5 ... 2.

Fig. 4 Adhesive wear curves h-L of the carbide composites investigated

The results of the adhesive wear test (as wear kinetic curves h - L) are presented in Fig. 4. They confirm the results of the blanking trials - the superiority of the developed TiC-based cermet (grade T75/14- H) over WC-based hardmetals (grade C13). These results refer to the existence of a correlation between the composite's blanking performance and its adhesive wear resistance.

B

A

Fig. 5 SEM image of the cemented carbides surface microstructure after blanking (A) and

testing of adhesive wear (B)

SEM studies of worn carbide composites surfaces confirm the conclusion stated above (see Fig. 5). They show that surface failure mechanisms occurring during adhesive wear and blanking are similar  in both cases failure (removal of material) appeared first in the binder. The distinctive nature of the worn surfaces confirms that binder extraction prevails in both cases 9, 10.

After removal of the binder (by extraction) the carbide phase loses the protective envelope (generating favourable compression stresses in the carbide), resulting in a drop of resistance to brittle failure (cracking/microcracking) 16. Thus, the increase in the composite's resistance to adhesive wear (removal of binder) results in the increase of its resistance to brittle failure (microchipping, cracking).

Fig. 6 shows the results of fatigue trials  the Wöhler plots of the advanced TiC-based cermet T75/14-H and WC-based hardmetal C13. The tested carbide composites exhibit an obvious decrease in strength S with an increase of loading cycles N during fatigue testing  they possess fatigue sensitivity (the slope S - N).

Fig. 6 Wöhler plots for hardmetal C13 and cermet T75/14-H

Although the transverse rupture strength (RTZ = S1 = 2.9 GPa) and cyclic strength at low cycles (N<103) of the WC hardmetal exceed those of the TiC-cermet (RTZ = 2.4 GPa), the fatigue limits at N>105 show an opposite result  the superiority of the cermet T75/14-H over the hardmetal (S7=1.7 GPa against 1.5 GPa). It means that the fatigue sensitivity, i.e the intensity in the decrease of strength with the increase in loading cycles (the slope S-N of the Wöhler plot) of TiC-cermet, is lower.

Table 2. Blanking performance N/D of carbide composites opposed to their properties (L1 - adhesive wear resistance,S3-7  fatigue sensitivity, S1=RTZ - transverse rupture strength, S3 and S7 - fatigue limit at 103 and 107 cycles, respectively

Grade

D,

m

N/D

X 106

strokes/mm

L1,

m/mm

S1=RTZ,

GPa

S3,

GPa

S7,

GPa

 S3-7,

GPa

C13

T75/14-H

3.0

1.8

160

300

2100

2500

2.9

2.4

2.3

2.0

1.5

1.7

0.75

0.30

High fatigue sensitivity refers to a remarkable fatigue damage of an alloy during cyclic loading (fatigue, blanking of a sheet metal). Table 2 demonstrates the blanking performance N/D (N - loading, blanking cycles, D- side wear of blanking tool) of composites, as opposed to the properties of inserts in the adhesion wear and fatigue testing conditions. Results refer to a correlation between the blanking performance of the carbide composite and its adhesive wear resistance on the one hand and its fatigue sensitivity on the other hand. The composite with a higher blanking performance T75/14-H possesses both a higher adhesive wear resistance and a lower fatigue sensitivity (higher resistance to fatigue damage).

The resistance of a material to a brittle failure is controlled by the level of the elastic strain energy transmitted during loading (monotonic, cyclic - fatigue, blanking) and storing preferably at the tips of flaws in the composite microstructure 16, 17. The elastic strain energy storing in a material during loading (fatigue, blanking wear) may be released either by the formation and propagation of cracks or by local plastic strain 16. The resistance of a carbide composite to brittle fracture depends therefore on its ability to undergo the local plastic strain (to absorb fracture energy by plastic strain).

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Fig. 7 X-ray diffraction patterns (diffractograms) of carbide phases of the TiC-based composite:

1 - before testing (etalon), 2 - fractured during monotonic loading; 3 - fractured

during cyclic loading.

Fig. 8 X-ray diffractogram of carbide phases of the WC-based carbide composite: 1 - before

testing, 2 - fractured during monotonic loading, 3 - fractured during cyclic loading

As stated, the plastic strain of a carbide composite takes place preferably in its ductile binder 11, 16. The XRD measurements performed in the present study on the fractured surfaces of the WC-based and TiC-based composites revealed alterations in their diffractograms - a decrease in the intensities (line peaks) and an increase in the broadness of the reflection lines of their carbide phases (Figs. 7 and 8). These alterations refer to changes in the fine structure (dispersity of micrograins and density of dislocation network) of the carbide phases 14, 15. It is known that such alterations in fine structure refer to local plastic strain 10. Thus, the ability of the carbide composite to absorb fracture energy depends on the plasticity of its both phases  the ductile binder and the "brittle" carbide.

Table 3. Decrease in the intensity (line peaks) and broadening of X -ray reflection lines from the carbide phases of the composites tested: Io, Im, Ic and Bo, Bm, Bc - intensity and broadness before (0), after monotonic (m) and after cyclic (c) loading

Grade

Carbide

line

Intensity,

Lin (Cps)

Decrease in intensity

Broadness B, °

Broadness, °

Io

Im

Ic

Io/Im

Io/Ic

Bo

Bm

Bc

Bm

Bc

C13

T75/14-H

WC 001

TiC 200

50

76

9.5

13

19

56

5.3

2.2

2.6

1.4

0.25

0.21

0.34

0.26

0.32

0.23

0.09

0.05

0.07

0.02

The results obtained (Figs.7, 8 and Table. 3) show that carbide phase plasticity depends on its composition and loading conditions. Under monotonic loading conditions WC appears to have a higher plasticity compared with TiC (Table 3, Io/Im). During cyclic loading (fatigue) the plasticity of the composite, carbide phases (WC and TiC) decreases  an embrittlement takes place.

The intensity in embrittlement depends on the composition of the alloy (carbide composite) and is more remarkable for a WC-based hardmetal (see Table 3 Io/Im and Io/Ic and line 3 in Figs. 7 and 8) compared to TiC-based cermets. The embrittlement (decrease in the plasticity of the composite / its phases) results in a decrease of strength with an increase of loading cycles during fatigue - an increase in the fatigue sensitivity of an alloy.

Thus, the superiority of the advanced TiC-based cermet (over the ordinary WC-based hardmetal), its higher blanking performance, may be related mainly to two properties - to higher adhesive wear resistance and higher resistance to fatigue damage  higher resistance to embrittlement during cyclic loading (to its lower fatigue sensitivity).

4. Conclusions

Performance in cyclic loading conditions  results of the functional test in the blanking of sheet metals and fatigue bending tests  of an advanced TiC-based cermet and that of a WC-based hardmetal (used in metalforming) were compared. As a result, the advantage of the cermet was found.

Reasonable correlations between the blanking performance (resistance to the side wear of the blanking tool) of the composite and its adhesive wear resistance and similarity in the surface failure morphology was found.

It was shown that the resistance of the carbide composite to fatigue failure during cyclic loading (blanking of sheet metal) is characterized by its fatigue sensitivity  intensity in the decrease of fatigue strength with an increase in loading cycles.

The failure of the carbide composite during monotonic and cyclic loading (bending fatigue, blanking of sheet metals) is preceded by local plastic strain taking place in its both phases - in the ductile binder and in the brittle carbide .

During cyclic loading (fatigue, blanking) the plasticity of the carbide composite (its phases) decreases and an embrittlement takes place.

The higher blanking performance of the TiC-based cermet (in relation to a hardmetal) results from its higher adhesive wear resistance and its lower fatigue sensitivity (as a result of its higher resistance to embrittlement).

Acknowledgements

This work was supported by the targeted financing project of the Estonian Ministry of Education and Research SF 0140062s08 and the Estonian Science Foundation grants No 5882 and 7889.