**2. Experimental procedure**

The chemical compositions of the FeCr particulates are given in Table 1. Composites containing 1 to 15wt% Fe/Cr particulates with an average particle size of about 20 m were prepared by a conventional P/M process, which involved the steps of mixing, cold isostatic pressing (1000 MPa), degassing and sintering according to the schedules in Table 2.

The hardness of the samples were measured in the range of 3 error band with HB hardness scale under 612.5 N load. In addition, the toughness of the samples was evaluated in the range of 0.6 error band using Charpy V-notch specimens. The tensile strength test samples were prepared upon ASTM E8-78 L0= 4d standard [27] and the tests were performed under Hounsfield type machine at room temperature, and by using a crosshead speed of 50 mm/min.

A pin-on disk apparatus was used for evaluating the abrasive wear resistance. For the abrasive wear tests, cylindrical billets of 12.5 mm diameter and 10 mm height were machined. Before the wear tests, each specimen was ground up to grade 1200 abrasive paper, making sure that the wear surface completely contacted the surface of the abrasive paper. Abrasive wear tests were carried out under dry sliding conditions by sliding the sample under an applied load of 10, 20, 30 and 40 N respectively over a grade 80 abrasive paper stuck to the grinding disk, which rotted at 320 rev min-1. A fixed track diameter of 160 mm was used in all tests, and the duration of abrading was 60 s. Each test was conducted using a fresh abrasive paper. For each test condition, at least three runs were performed. Wear rates were obtained by determining the weight loss of the samples before and after wear tests.

Samples for microscopic examination were prepared by standard metaleographic procedures; they were then etched with %1 nital reagent and examined by optical and scanning electron microscopy (SEM). For determination of the wear mechanism of the Fe alloy and its composites, the worn surfaces and debris were examined by scanning electron microscopy, where the samples were gold coated prior to examinations.



**Table 2.** Processing schedules of the powder metallurgy Fe composites.

#### **3. Results and discussions**

#### **3.1. Microsturucture**

4 Tribology in Engineering

Most studies [17-21] indicated that the wear resistance of MMCSs manufactured by the P/M and/or casting techniques increased with increasing volume fraction of reinforcement particulates. The wear resistance of the composite decreased with increasing reinforcement above a certain level. Jha *et al*. [26] indicated that the wear rates increased with increasing

In this study, the Fe base P/M composites are reinforced with FeCr carbide complexes, soft graphite, and Cu particles to improve wear resistance benefiting from the advantage of both the energy absorption properties of the soft matrix phases and the wear resistance of the hard carbide phases. With this aim, we investigated the microstructures, wear properties and some mechanical properties (surface hardness, tensile strength and toughness of Fe base MMCSs) by using scanning electron microscopy (SEM), surface hardness (HB), tensile

The chemical compositions of the FeCr particulates are given in Table 1. Composites containing 1 to 15wt% Fe/Cr particulates with an average particle size of about 20 m were prepared by a conventional P/M process, which involved the steps of mixing, cold isostatic

The hardness of the samples were measured in the range of 3 error band with HB hardness scale under 612.5 N load. In addition, the toughness of the samples was evaluated in the range of 0.6 error band using Charpy V-notch specimens. The tensile strength test samples were prepared upon ASTM E8-78 L0= 4d standard [27] and the tests were performed under Hounsfield type machine at room temperature, and by using a crosshead speed of 50

A pin-on disk apparatus was used for evaluating the abrasive wear resistance. For the abrasive wear tests, cylindrical billets of 12.5 mm diameter and 10 mm height were machined. Before the wear tests, each specimen was ground up to grade 1200 abrasive paper, making sure that the wear surface completely contacted the surface of the abrasive paper. Abrasive wear tests were carried out under dry sliding conditions by sliding the sample under an applied load of 10, 20, 30 and 40 N respectively over a grade 80 abrasive paper stuck to the grinding disk, which rotted at 320 rev min-1. A fixed track diameter of 160 mm was used in all tests, and the duration of abrading was 60 s. Each test was conducted using a fresh abrasive paper. For each test condition, at least three runs were performed. Wear rates were obtained by determining the weight loss of the samples before and after

Samples for microscopic examination were prepared by standard metaleographic procedures; they were then etched with %1 nital reagent and examined by optical and scanning electron microscopy (SEM). For determination of the wear mechanism of the Fe alloy and its composites, the worn surfaces and debris were examined by scanning electron

microscopy, where the samples were gold coated prior to examinations.

pressing (1000 MPa), degassing and sintering according to the schedules in Table 2.

reinforcement volume fraction in the P/M sintered soft matrix alloys.

testing, Charpy V-notch impact and abrasive wear tests.

**2. Experimental procedure** 

mm/min.

wear tests.

The microstructures of the composites with FeCr reinforcement were investigated and optical micrograph of the sample S1 is given in Figure 1. It was seen that, the microstructure of the FeCr reinforced MMCSs consist of ferrite matrix with dispersed FeCr particulates. The addition of graphite to the composite with FeCr particulates formed different phases (Figure

**Figure 1.** Optical micrographs of the sample S1

2-Table 3). Depending on graphite amount, pearlite phase started to form around graphite particles, and increasing the amount of graphite increased the ratio of pearlite phase and M3C carbides. Graphite grains were formed in the pearlite structure in samples with 1wt% graphite supplement. On the other hand, by increasing graphite content to 2wt%, ledeburitic structure has been formed in grain boundaries besides formation of M3C carbides at grain boundaries and toward center of grains (Figure 3). The microstructures of the samples having soft copper supplement in the range 0.5-2 wt% with graphite (0.5wt%) and FeCr (5wt%) particulates were found near to each other, but their microstructures was different than the samples having a structure without copper supplement (Figure 4-5 and Table 4.).

Effect of FeCr Intermetallic on Wear Resistance of Fe-Based Composites 7

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15

S8 200 201 S9 101 200 110 201 S10 101 200 002 210 201 111 110 210

S8 211 413 101 S9 222 211 413 101 S10 222 211 413 101

S8 110 200 211 220 310 222 S9 110 200 211 220 310 222

S9 002 100 110 S10 002 100 110

S9 111 200 222

S10 110 200 211 220 310 222

S8 111 200 220 222

S10 111 200 220 311 222

1 2 3 4 5 6 7 8 9 10

M23C6 111 200 220 211 310 222 C 002 110 300 400

The results of surface hardness, toughness, tensile strength and wear resistance tests are summarized in Table 5. The hardness of the FeCr reinforced samples (S1-S6) increased by FeCr reinforcement (Figure 6.a). The reason for this increase in hardness can be attributed to the increase in wt% of FeCr reinforcement and the diffusion of dissolved Cr atoms into the matrix. Moreover, the addition of graphite with FeCr particulates increased hardness significantly (Figure 6.b). The reason is thought to be due to formation of pearlite phase in matrix, and as well due to the ledeburite, M3C2, and phases (Figure 4-Table 3). In addition to the X-ray diffraction, EDS analysis form these phases are taken and it was detected as; the amount of Cr is 6wt.% and 8-12 wt.% in M3C and ledeburitic structure, respectively. Furthermore, the hardness data of the samples S7-S10 also showed well agreement with the

Phases Pikes

S8 002 110

Phases Pikes

MC3 020 201 011 110

M7C3 100 102 110

Fe 111 110 200 211 220 210 222

M3C

M7C3

Fe

C

M23C6

**Table 3.** X ray pikes of sample S10

**Table 4.** X ray pikes of S13

**3.2. Mechanical testing** 

presence of the phases obtained (Table 3).

**Figure 2.** X-ray difractom of the sample S10

**Figure 3.** Optical micrographs of the sample S10.

**Figure 4.** X-ray difractom of the samples S13


**Table 3.** X ray pikes of sample S10

6 Tribology in Engineering

**Figure 2.** X-ray difractom of the sample S10

**Figure 3.** Optical micrographs of the sample S10.

**Figure 4.** X-ray difractom of the samples S13


**Table 4.** X ray pikes of S13

#### **3.2. Mechanical testing**

The results of surface hardness, toughness, tensile strength and wear resistance tests are summarized in Table 5. The hardness of the FeCr reinforced samples (S1-S6) increased by FeCr reinforcement (Figure 6.a). The reason for this increase in hardness can be attributed to the increase in wt% of FeCr reinforcement and the diffusion of dissolved Cr atoms into the matrix. Moreover, the addition of graphite with FeCr particulates increased hardness significantly (Figure 6.b). The reason is thought to be due to formation of pearlite phase in matrix, and as well due to the ledeburite, M3C2, and phases (Figure 4-Table 3). In addition to the X-ray diffraction, EDS analysis form these phases are taken and it was detected as; the amount of Cr is 6wt.% and 8-12 wt.% in M3C and ledeburitic structure, respectively. Furthermore, the hardness data of the samples S7-S10 also showed well agreement with the presence of the phases obtained (Table 3).

Effect of FeCr Intermetallic on Wear Resistance of Fe-Based Composites 9

words, the amount of Cr in martensitic and bainitic structures increased with increasing the

**Figure 6.** The relationship between hardness and weight loss as function of (a) FeCr (b) graphite (c)

amount of copper supplement.

copper concentration

**Figure 5.** Optical micrographs of the sample S13


**Table 5.** The mean values and error bands of results of surface hardness, toughness and wear resistance tests

The effect of copper supplement with graphite and FeCr particulates on the hardness of the samples was detected for the samples namely S11, S12 and S13. The relationship between hardness and wear resistance was given in Figure 6.c as function of Cu concentration. As seen from the figure, Cu supplement increased hardness considerably. The increase in hardness is probably due to the microstructural change, and the reason for formation of martensitic and bainitic structures are due to presence of alloying elements. C, Cr and Cu elements in the structure decrease formation temperature of martensite (Ms) and bainite (Bs) and they provoke formation of these phases [28,29]. It was detected that, copper is present as 0.05-0.25, 1.43-2.87, 1.98-4.94 wt% in martensitic+bainitic zone, respectively, and Cr is detected in the range of 1.98-4.94 wt% Cr in the same samples by EDS analysis. In other words, the amount of Cr in martensitic and bainitic structures increased with increasing the amount of copper supplement.

8 Tribology in Engineering

tests

**Figure 5.** Optical micrographs of the sample S13

Composition Hardness

Weight Loss (mg)

10 N 20 N 30 N

T Strength MPa

HB

S1 Fe 55,75 486 27 0,57 0,87 1,12 S2 Fe+1wt%FeCr 64,4 336 24 0,55 0,83 1,09 S3 Fe+3wt%FeCr 72 384 32 0,6 1,93 1,235 S4 Fe+5wt%feCr 77,66 207,5 41,5 0,7 1,023 1,254 S5 Fe+7wt%FeCr 94 48 48 0,95 1,1 1,51 S6 Fe+10wt%FeCr 116 40,95 58,5 1,2 1,4 1,859 S7 Fe+5wt%FeCr+ 0,25wt% Graphite 79 492,64 61,58 0,78 0,87 1,12 S8 Fe+5wt%FeCr+ 0,5wt% Graphite 95 504 72 0,61 0,75 0,98 S9 Fe+5wt%FeCr+ 1wt% Graphite 121,75 420 84 0,57 0,7 0,94 S10 Fe+5wt%FeCr+ 2wt% Graphite 281 311,5 89 0,18 0,27 0,31 S11 Fe+5wt%FeCr+0,5wt%Graphite+%0,5wt%Cu 214 429 78 0,31 0,45 0,5 S12 Fe+5wt%FeCr+0,5wt%Graphite+1wt%Cu 220 346,4 86,6 0,29 0,4 0,45 S13 Fe+5wt%FeCr+0,5wt%Graphite+2wt%Cu 224 241,975 96,79 0,21 0,34 0,4 **Table 5.** The mean values and error bands of results of surface hardness, toughness and wear resistance

The effect of copper supplement with graphite and FeCr particulates on the hardness of the samples was detected for the samples namely S11, S12 and S13. The relationship between hardness and wear resistance was given in Figure 6.c as function of Cu concentration. As seen from the figure, Cu supplement increased hardness considerably. The increase in hardness is probably due to the microstructural change, and the reason for formation of martensitic and bainitic structures are due to presence of alloying elements. C, Cr and Cu elements in the structure decrease formation temperature of martensite (Ms) and bainite (Bs) and they provoke formation of these phases [28,29]. It was detected that, copper is present as 0.05-0.25, 1.43-2.87, 1.98-4.94 wt% in martensitic+bainitic zone, respectively, and Cr is detected in the range of 1.98-4.94 wt% Cr in the same samples by EDS analysis. In other

Toughness J Cm2

**Figure 6.** The relationship between hardness and weight loss as function of (a) FeCr (b) graphite (c) copper concentration

Reinforcement of the FeCr carbides increased toughness of the samples (S1-S6) significantly (Figure 7.a). However, the toughness of the samples having graphite was changed, and an optimum point for the amount of graphite was found. At the beginning, graphite increased toughness, however after 0.5 wt.% graphite additions, the amount of graphite decreased toughness (Figure 7.b). This decrease is attributed to the presence of graphite particles, hard intermetallic phases and the diffusion of Cr into the matrix producing a brittle structure.

Effect of FeCr Intermetallic on Wear Resistance of Fe-Based Composites 11

**Figure 7.** The relationship between toughness and weight loss as function of (a) FeCr (b) graphite (c)

copper concentration

The weight loss of the samples S7, S8, S9 and S10 decreased with the increase in the amount of graphite. Investigations on microstructure of the samples having graphite supplement in the range 0.25 to 2 wt% (with 5wt% FeCr particulates) have showed that additional phases were formed. Moreover, the increase in the amount of graphite also decreased the size of FeCr particulates. It was conjectured that the decrease of particulate size improved wear resistance. Because, the good bonding between the composite constituents avoids third body abrasion and allow to FeCr particulates to act as load-bearing elements of the composite. Investigations on the toughness of the samples having copper show that copper addition decreased toughness (Figure 7.c), because copper increased the amount of Cr in all of the phases, and provided formation of martensite and bainite phases. Furthermore it increased the diffusion rate and decreased the size of carbides

The change in weight loss vs. load of the samples S1-S6 is given in Figure 8.a. For all loads, the highest weight loss obtained for the sample S6 and the sample S1 gave the lowest weight loss. Investigations on the microstructure of the sample S6 show that, the matrix of S6 was constituted from ductile ferrite phase. Hence, it is conjectured that FeCr particulates were easily pulled out during wear. Also, within craters (*i.e.,* where flakes of material cracked and wore away) FeCr particulates protruded from the surface. Particularly the sample S10 has shown the lowest weight loss and a different wear behavior. On the other hand for all loads the wear rate of the samples S6-S9 are near to each other (Figure 8.b).

Copper was added to the matrix of the samples S11-S13 having FeCr particulates (5wt.%) and graphite (0.5wt.%) together to decrease amount of porosity and friction coefficient. It was observed that copper increased hardness, tensile strength, but decreased toughness and weight loss. The change of weight loss with load has given in Figure 8.c. From the figure it is seen that the effect of load decreased with copper supplement. The relationship between hardness and wear resistance of the samples are given in Figure 9 for 30 N load. The wear tests show that there isn't correlations between wear resistance and hardness for the samples (S1-S6) having FeCr reinforcements under abrasive tests over 80 grade abrasives. As the hardness increased, the weight loss increased. On the other hand, addition of graphite to the matrix of the samples (S7-S10) increased hardness and decreased weight loss, but toughness of the samples didn't changed parallel to the weight loss of the samples. Because, the microstructure of the matrix has been changed by diffusion of carbon atoms into the matrix, and besides shrinkage of carbide particulates have been seen. Furthermore, the Cu supplements have shown that the wear rate of samples S11, S12, S13 changed proportionally with surface hardness.

Reinforcement of the FeCr carbides increased toughness of the samples (S1-S6) significantly (Figure 7.a). However, the toughness of the samples having graphite was changed, and an optimum point for the amount of graphite was found. At the beginning, graphite increased toughness, however after 0.5 wt.% graphite additions, the amount of graphite decreased toughness (Figure 7.b). This decrease is attributed to the presence of graphite particles, hard intermetallic phases and the diffusion of Cr into the matrix producing a brittle structure.

The weight loss of the samples S7, S8, S9 and S10 decreased with the increase in the amount of graphite. Investigations on microstructure of the samples having graphite supplement in the range 0.25 to 2 wt% (with 5wt% FeCr particulates) have showed that additional phases were formed. Moreover, the increase in the amount of graphite also decreased the size of FeCr particulates. It was conjectured that the decrease of particulate size improved wear resistance. Because, the good bonding between the composite constituents avoids third body abrasion and allow to FeCr particulates to act as load-bearing elements of the composite. Investigations on the toughness of the samples having copper show that copper addition decreased toughness (Figure 7.c), because copper increased the amount of Cr in all of the phases, and provided formation of martensite and bainite phases. Furthermore it increased

The change in weight loss vs. load of the samples S1-S6 is given in Figure 8.a. For all loads, the highest weight loss obtained for the sample S6 and the sample S1 gave the lowest weight loss. Investigations on the microstructure of the sample S6 show that, the matrix of S6 was constituted from ductile ferrite phase. Hence, it is conjectured that FeCr particulates were easily pulled out during wear. Also, within craters (*i.e.,* where flakes of material cracked and wore away) FeCr particulates protruded from the surface. Particularly the sample S10 has shown the lowest weight loss and a different wear behavior. On the other hand for all loads

Copper was added to the matrix of the samples S11-S13 having FeCr particulates (5wt.%) and graphite (0.5wt.%) together to decrease amount of porosity and friction coefficient. It was observed that copper increased hardness, tensile strength, but decreased toughness and weight loss. The change of weight loss with load has given in Figure 8.c. From the figure it is seen that the effect of load decreased with copper supplement. The relationship between hardness and wear resistance of the samples are given in Figure 9 for 30 N load. The wear tests show that there isn't correlations between wear resistance and hardness for the samples (S1-S6) having FeCr reinforcements under abrasive tests over 80 grade abrasives. As the hardness increased, the weight loss increased. On the other hand, addition of graphite to the matrix of the samples (S7-S10) increased hardness and decreased weight loss, but toughness of the samples didn't changed parallel to the weight loss of the samples. Because, the microstructure of the matrix has been changed by diffusion of carbon atoms into the matrix, and besides shrinkage of carbide particulates have been seen. Furthermore, the Cu supplements have shown that the wear rate of samples S11, S12, S13 changed proportionally

the diffusion rate and decreased the size of carbides

with surface hardness.

the wear rate of the samples S6-S9 are near to each other (Figure 8.b).

**Figure 7.** The relationship between toughness and weight loss as function of (a) FeCr (b) graphite (c) copper concentration

Effect of FeCr Intermetallic on Wear Resistance of Fe-Based Composites 13

hardness of the matrix, and particulates indicated that they act as load-bearing elements more efficiently than ceramics. Furthermore hard abrasive ceramic reinforcements, such as SiC, have the deleterious effect of wearing the counterface more than the unreinforced material does [30,31]. In addition, as cracks might propagate through the matrix/ceramic reinforcement interface [32,30] pulled out ceramic particles may act as third-body abrasion elements [32] of both specimens and counterfaces, worsening wear behavior of

**Figure 9.** The relationship between hardness and weight loss (30 N load and 80 grade abrasive).

*Department of Material and Metallurgical Engineering Frat University Elaziğ, Turkey* 

The wear tests applied over 80 grade abrasive papers have shown that the weight loss of the MMCs having only FeCr particulates increased with increase of FeCr ratio. However, increase in wt.% of FeCr particulates increased hardness, toughness and yield strength. Graphite supplement with FeCr particulates have formed additional phases, decreased size of FeCr particulates and increased matrix hardness. Hence, weight loss decreased, and increasing graphite amount increased hardness linearly. On the other hand, toughness of the samples having graphite additives decreased after 0.5wt.% graphite. Nevertheless, the wear rate of the samples were changed accordingly to load, but the wear rate of S10 didn't changed in a considerable amount with load, and its rate was far low than other samples. Samples with copper additive have shown an increase in hardness, tensile strength with Cu amount. However, toughness was decreased with weight loss. In addition the dependence of the weight loss of the samples with copper to the load decreased with copper addition.

composite/counterface system.

**4. Conclusion** 

**Author details** 

Corresponding Author

and M. Aksoy

S.O. Ylmaz\*

 \*

**Figure 8.** Wear rate vs. load for the samples (a) S1-S6 (b) S7-S10 (c) S11-S13

A qualitative difference was found between tracks of Fe/FeCr/graphite-Fe/FeCr/Cu and Fe/FeCr specimens, suggesting less sensitivity to the load of the former. This delay of the load effect can be attributed to the presence of FeCr particulates, size of particulates and hardness of the matrix, and particulates indicated that they act as load-bearing elements more efficiently than ceramics. Furthermore hard abrasive ceramic reinforcements, such as SiC, have the deleterious effect of wearing the counterface more than the unreinforced material does [30,31]. In addition, as cracks might propagate through the matrix/ceramic reinforcement interface [32,30] pulled out ceramic particles may act as third-body abrasion elements [32] of both specimens and counterfaces, worsening wear behavior of composite/counterface system.

**Figure 9.** The relationship between hardness and weight loss (30 N load and 80 grade abrasive).

## **4. Conclusion**

12 Tribology in Engineering

**Figure 8.** Wear rate vs. load for the samples (a) S1-S6 (b) S7-S10 (c) S11-S13

A qualitative difference was found between tracks of Fe/FeCr/graphite-Fe/FeCr/Cu and Fe/FeCr specimens, suggesting less sensitivity to the load of the former. This delay of the load effect can be attributed to the presence of FeCr particulates, size of particulates and The wear tests applied over 80 grade abrasive papers have shown that the weight loss of the MMCs having only FeCr particulates increased with increase of FeCr ratio. However, increase in wt.% of FeCr particulates increased hardness, toughness and yield strength. Graphite supplement with FeCr particulates have formed additional phases, decreased size of FeCr particulates and increased matrix hardness. Hence, weight loss decreased, and increasing graphite amount increased hardness linearly. On the other hand, toughness of the samples having graphite additives decreased after 0.5wt.% graphite. Nevertheless, the wear rate of the samples were changed accordingly to load, but the wear rate of S10 didn't changed in a considerable amount with load, and its rate was far low than other samples. Samples with copper additive have shown an increase in hardness, tensile strength with Cu amount. However, toughness was decreased with weight loss. In addition the dependence of the weight loss of the samples with copper to the load decreased with copper addition.
