*2.2.3 Structural characterization of the 97MXC coatings*

The morphology and metallographic structure of 96MXC deposits were investigated on micrographs obtained using the FEI - Quanta 200 3D scanning electron microscope (SEM). The investigations were performed on the cross section. The samples, measuring 10 mm x 10 mm x 10 mm, were obtained by cutting, after which there were incorporated into epoxy resin, sanded and polished. For the metallographic characterization of the compounds present in the analyzed coatings structures, the surfaces were chemically attacked with Vilella reagent (a solution with 10 ml of HF, 5 ml of HNO3, and 85 ml of H2O) for 10 min. After preparation, the samples were analyzed with the help of the SEM microscope.

**Figure 12** presents two representative SE (secondary electron) images of the 97MXC deposits, obtained by arc spray process (I - 220 A), at different values of the compressed air pressure passing through the primary circuit: pprimary air = 5.5 bar; and pprimary air = 6.5 bar.

The deposits from **Figure 12** present a heterogeneous microstructure formed by flattened lamellas (usually called splats), oriented parallel to the substrate, polygonal formations, unmelted spherical particles and pores - being specific microstructure to the deposits obtained by arc spray process. In the cross section of the samples are observed several variations of the structural elements brightness (various shades of gray), an aspect that suggests the inhomogeneity of the chemical composition.

XRD patterns, EDX analyses and SE images allow the identification of the various phases of the coatings. Thus, the matrix formed by light gray metal splats corresponds to FeCr and FeW phases, the dark gray flattened splats correspond to hard Fe2B phases, at the limit of the splats some interstitial oxides appear and the areas with dark contrast correspond to the pores.

#### **Figure 12.**

*SEM images of 97MXC coatings obtained under the following conditions, I = 220A, SOD = 110 mm, U = 32 V: (a) pprimary air = 5.5 bar; (b) pprimary air = 6.5 bar.*

**149**

**Figure 13.**

*Hard Alloys with High Content of WC and TiC—Deposited by Arc Spraying Process*

In the micrographs taken on the cross section, light-colored and bright polygonal formations appear, which correspond to the W-rich hard phase (WC) embedded in a matrix composed of eta type carbide (FeW)xC of light gray color. There are also present some dark gray polygonal formations which corresponds to titanium carbide (TiC). W-rich alloyed areas have a heterogeneous distribution, and inside them are formed eutectic phases of WC and W2C type - similar to the results

It can be observed that at low values of the compressed air pressure passing through the primary circuit - see **Figure 12a**, in the deposit are obtained particles of larger dimensions, compared to the particle sizes presented in **Figure 12b**, which have a flattened shape. This aspect is explained by the fact that by increasing the pressure of the compressed air, the speed of the gas jet and implicitly the impact speed of the particles increase. The presence of a small quantity of unmelted particles inside the coating suggests that the temperature of the particles at the impact

These aspects are confirmed by the investigations performed at the interface between the W-rich eutectic carbides (WC) and the metallic matrix based on Fe - see **Figure 13**. The deposits obtained at high values of electric current intensity present, around the polygonal eutectic carbides, at the interface between the W-rich phase and the Fe-based matrix, some transition areas (the luminous phase to darkness), presented in **Figure 13a,c**. The EDX analyses carried out in the dark area

We found that the width of the transition area decreases until it disappears with the decrease of the current intensity that supplies the electric arc during the

*Cross-section images taken by SEM microscopy showing the embedment polygonal carbide WC at pair secundary = 6.5 bar: (a) I = 250A; (b) I = 200A; (c) detail figure a; (d) EDX spot analysis.*

moment together with the spray distance were optimally chosen.

indicate the presence of a zone rich alloyed in Fe.

*DOI: http://dx.doi.org/10.5772/intechopen.94605*

reported by Tillmann et al., [44].

*Hard Alloys with High Content of WC and TiC—Deposited by Arc Spraying Process DOI: http://dx.doi.org/10.5772/intechopen.94605*

In the micrographs taken on the cross section, light-colored and bright polygonal formations appear, which correspond to the W-rich hard phase (WC) embedded in a matrix composed of eta type carbide (FeW)xC of light gray color. There are also present some dark gray polygonal formations which corresponds to titanium carbide (TiC). W-rich alloyed areas have a heterogeneous distribution, and inside them are formed eutectic phases of WC and W2C type - similar to the results reported by Tillmann et al., [44].

It can be observed that at low values of the compressed air pressure passing through the primary circuit - see **Figure 12a**, in the deposit are obtained particles of larger dimensions, compared to the particle sizes presented in **Figure 12b**, which have a flattened shape. This aspect is explained by the fact that by increasing the pressure of the compressed air, the speed of the gas jet and implicitly the impact speed of the particles increase. The presence of a small quantity of unmelted particles inside the coating suggests that the temperature of the particles at the impact moment together with the spray distance were optimally chosen.

These aspects are confirmed by the investigations performed at the interface between the W-rich eutectic carbides (WC) and the metallic matrix based on Fe - see **Figure 13**. The deposits obtained at high values of electric current intensity present, around the polygonal eutectic carbides, at the interface between the W-rich phase and the Fe-based matrix, some transition areas (the luminous phase to darkness), presented in **Figure 13a,c**. The EDX analyses carried out in the dark area indicate the presence of a zone rich alloyed in Fe.

We found that the width of the transition area decreases until it disappears with the decrease of the current intensity that supplies the electric arc during the

#### **Figure 13.**

*Cross-section images taken by SEM microscopy showing the embedment polygonal carbide WC at pair secundary = 6.5 bar: (a) I = 250A; (b) I = 200A; (c) detail figure a; (d) EDX spot analysis.*

*Welding - Modern Topics*

of C-poor compounds such as W2C.

and pprimary air = 6.5 bar.

*2.2.3 Structural characterization of the 97MXC coatings*

areas with dark contrast correspond to the pores.

the samples were analyzed with the help of the SEM microscope.

FeB, Cr2B, Fe2O3 and Fe3O4. Peaks of WC and W2C are present in all three coatings, formed as a result of decomposition during the thermal spraying process, similar to the results reported by He et al. [43]. In additional to the eutectic phases of WC, W2C and TiC, alloyed solid solutions of γ(Fe, Ni) and γ(Ni, Cr) were also identified. It is noted that the intensity of the W2C peak increases with increasing current intensity. It is also suggested that the high temperature of the electric arc favors the decomposition of the WC carbides into single elements, respectively the formation

The morphology and metallographic structure of 96MXC deposits were investigated on micrographs obtained using the FEI - Quanta 200 3D scanning electron microscope (SEM). The investigations were performed on the cross section. The samples, measuring 10 mm x 10 mm x 10 mm, were obtained by cutting, after which there were incorporated into epoxy resin, sanded and polished. For the metallographic characterization of the compounds present in the analyzed coatings structures, the surfaces were chemically attacked with Vilella reagent (a solution with 10 ml of HF, 5 ml of HNO3, and 85 ml of H2O) for 10 min. After preparation,

**Figure 12** presents two representative SE (secondary electron) images of the 97MXC deposits, obtained by arc spray process (I - 220 A), at different values of the compressed air pressure passing through the primary circuit: pprimary air = 5.5 bar;

The deposits from **Figure 12** present a heterogeneous microstructure formed by flattened lamellas (usually called splats), oriented parallel to the substrate, polygonal formations, unmelted spherical particles and pores - being specific microstructure to the deposits obtained by arc spray process. In the cross section of the samples are observed several variations of the structural elements brightness (various shades of gray), an aspect that suggests the inhomogeneity of the chemical composition. XRD patterns, EDX analyses and SE images allow the identification of the various phases of the coatings. Thus, the matrix formed by light gray metal splats corresponds to FeCr and FeW phases, the dark gray flattened splats correspond to hard Fe2B phases, at the limit of the splats some interstitial oxides appear and the

*SEM images of 97MXC coatings obtained under the following conditions, I = 220A, SOD = 110 mm, U = 32 V:* 

**148**

**Figure 12.**

*(a) pprimary air = 5.5 bar; (b) pprimary air = 6.5 bar.*

spraying process. As an example, in **Figure 13b** is presented polygonal WC carbide where the absence of the transition zone is observed.

It can be observed that the W content decreases and the Fe amount increases in the transition area, as the distance to the eutectic polygonal carbide increases (see **Figure 13d**). These aspects suggest the fact that, by increasing the temperature of the sprayed particles and due to the increase of the electric arc intensity, the appearance of a transition zone between the polygonal eutectic carbides' WC type and the metallic matrix is favored. It can be suggested that high particle temperatures permit a better integration of the unmelted polygonal eutectic carbides into the matrix of Fe.

### *2.2.4 Microhardness of the 97MXC coatings*

The hardness is the capacity of a piece to oppose the tendency to destroy the surface coatings by another piece, which acts on it with localized pressures on very small areas. In determining the hardness of materials, account shall be taken of the size of the traces produced by a penetrating piece, characterized by a certain shape and size and of the force acting on it. The hardness of a material is appreciated by the value of some conventional characteristics, obtained after some non-destructive tests.

Because the porous structure of the sprayed coatings does not permit the exactly determination of the hardness by conventional methods, in order to carry out investigations regarding the micro-hardness of the 97MXC deposits, we considered that the most suitable method is the Vickers method. The microhardness represents the Vickers hardness of some elements from the metallographic structure (phases, structural constituents, inclusions, etc.) and of some very thin coatings. The Vickers microhardness values presented in our study were determined using the CV - 400DAT digital microdurimeter, produced by CV Instruments, with a 100 g load, for 10s.

In order to establish the microhardness of the 97MXC coatings, we performed 5 determinations in points located at a minimum distance of 0,5 mm from each other, arranged on the transverse direction of the coating - according to the norm SR EN ISO 14923/2004.

The average values of the microhardness of the 97MXC deposits, produced in different experimental conditions are presented in **Figure 14**.

As the data presented in **Figure 14** show, the micro-hardness of 97MXC coatings is relatively high. This aspect is due both to the phases, rich alloyed in W or Ti and to the chemical compounds based on Fe2Cr or of the eta type carbide (FeW)xC - which

**151**

**Figure 15.**

*General view of testing machine.*

*Hard Alloys with High Content of WC and TiC—Deposited by Arc Spraying Process*

ness of the deposits is relatively low of HV100 = 664 ± 42 N/mm2

determines the increase of the microhardness of 97MXC deposits.

microhardness is relatively high. However, it is observed that the micro hardness of the 97MXC coatings varies in limits of up to 125 units with the pressure of the compressed air passing through the primary circuit and with the intensity of the spray current. Thus, for low values of the electric current intensity, the microhard-

microhardness of the depositions obtained at values of the electric current intensity

homogeneity of depositions obtained at high values of the electric current intensity

Microhardness investigations were also performed on the W, Ti and Cr hard phases. In **Figure 13b** and **c** are presented traces of the Vickers penetrator produced on the W-rich phase, in the area of the Fe phase highly alloyed with W and C (at interface) - **Figure 13b**, as well as in the low alloyed Fe phase - **Figure 13b**.

For the light-colored phases, the average microhardness value was 2842 N/mm2

and inside the Fe-rich phase, the average microhardness value was 473 N/mm<sup>2</sup>

phase, positioned between the flattened particles) had an average microhardness

interface level between the W-rich eutectic phase and the metal matrix, rich alloyed in Fe, obtained the increasing the electric arc intensity, contains complex compounds based on Fe, W and C of (FeW)xC type, whose hardness is relatively high.

The wear behavior of the 97MCX coatings was evaluated by sliding wear tests

A general view of the AMSLER machine is given in **Figure 15**. The AMSLER machine was equipped with a data acquisition system based on tensometric strain gauges system [45], calibrated by deadweights method. The interface of the data acquisition system was realized in LabVIEW program [46]. The coated parallelepi-

Each coated sample was tested twice: at 20 N and 40 N normal load. The speed of the disk was kept constant, N = 100 rpm. The testing time was 3600 s. No lubri-

ped sample on rotating steel disc testing arrangement is presented detail.

cant was used. Before each test, samples were cleaned with acetone.

. It can be suggested the fact that the transition phase, formed at the

the interface level, the average microhardness value was 1387 N/mm2

phases had an average microhardness of 845 N/mm2

*2.2.5 The wear behavior of the 97MCX ultra hard coatings*

compared to the

,

. At

. The TiC–rich

, the Cr-rich phases had an

. It can be affirmed that the high degree of

and those of interstitial oxides (the dark

*DOI: http://dx.doi.org/10.5772/intechopen.94605*

higher than HV100 = 789 ± 32 N/mm2

average microhardness of 636 N/mm<sup>2</sup>

conducted on an Amsler type system.

of 258 N/mm<sup>2</sup>

**Figure 14.** *HV100 microhardness of 97MXC coatings.*

*Hard Alloys with High Content of WC and TiC—Deposited by Arc Spraying Process DOI: http://dx.doi.org/10.5772/intechopen.94605*

microhardness is relatively high. However, it is observed that the micro hardness of the 97MXC coatings varies in limits of up to 125 units with the pressure of the compressed air passing through the primary circuit and with the intensity of the spray current. Thus, for low values of the electric current intensity, the microhardness of the deposits is relatively low of HV100 = 664 ± 42 N/mm2 compared to the microhardness of the depositions obtained at values of the electric current intensity higher than HV100 = 789 ± 32 N/mm2 . It can be affirmed that the high degree of homogeneity of depositions obtained at high values of the electric current intensity determines the increase of the microhardness of 97MXC deposits.

Microhardness investigations were also performed on the W, Ti and Cr hard phases. In **Figure 13b** and **c** are presented traces of the Vickers penetrator produced on the W-rich phase, in the area of the Fe phase highly alloyed with W and C (at interface) - **Figure 13b**, as well as in the low alloyed Fe phase - **Figure 13b**.

For the light-colored phases, the average microhardness value was 2842 N/mm2 , and inside the Fe-rich phase, the average microhardness value was 473 N/mm<sup>2</sup> . At the interface level, the average microhardness value was 1387 N/mm2 . The TiC–rich phases had an average microhardness of 845 N/mm2 , the Cr-rich phases had an average microhardness of 636 N/mm<sup>2</sup> and those of interstitial oxides (the dark phase, positioned between the flattened particles) had an average microhardness of 258 N/mm<sup>2</sup> . It can be suggested the fact that the transition phase, formed at the interface level between the W-rich eutectic phase and the metal matrix, rich alloyed in Fe, obtained the increasing the electric arc intensity, contains complex compounds based on Fe, W and C of (FeW)xC type, whose hardness is relatively high.

#### *2.2.5 The wear behavior of the 97MCX ultra hard coatings*

The wear behavior of the 97MCX coatings was evaluated by sliding wear tests conducted on an Amsler type system.

A general view of the AMSLER machine is given in **Figure 15**. The AMSLER machine was equipped with a data acquisition system based on tensometric strain gauges system [45], calibrated by deadweights method. The interface of the data acquisition system was realized in LabVIEW program [46]. The coated parallelepiped sample on rotating steel disc testing arrangement is presented detail.

Each coated sample was tested twice: at 20 N and 40 N normal load. The speed of the disk was kept constant, N = 100 rpm. The testing time was 3600 s. No lubricant was used. Before each test, samples were cleaned with acetone.

**Figure 15.** *General view of testing machine.*

*Welding - Modern Topics*

matrix of Fe.

ISO 14923/2004.

spraying process. As an example, in **Figure 13b** is presented polygonal WC carbide -

It can be observed that the W content decreases and the Fe amount increases in the transition area, as the distance to the eutectic polygonal carbide increases (see **Figure 13d**). These aspects suggest the fact that, by increasing the temperature of the sprayed particles and due to the increase of the electric arc intensity, the appearance of a transition zone between the polygonal eutectic carbides' WC type and the metallic matrix is favored. It can be suggested that high particle temperatures permit a better integration of the unmelted polygonal eutectic carbides into the

The hardness is the capacity of a piece to oppose the tendency to destroy the surface coatings by another piece, which acts on it with localized pressures on very small areas. In determining the hardness of materials, account shall be taken of the size of the traces produced by a penetrating piece, characterized by a certain shape and size and of the force acting on it. The hardness of a material is appreciated by the value of

Because the porous structure of the sprayed coatings does not permit the exactly

In order to establish the microhardness of the 97MXC coatings, we performed 5 determinations in points located at a minimum distance of 0,5 mm from each other, arranged on the transverse direction of the coating - according to the norm SR EN

The average values of the microhardness of the 97MXC deposits, produced in

As the data presented in **Figure 14** show, the micro-hardness of 97MXC coatings is relatively high. This aspect is due both to the phases, rich alloyed in W or Ti and to the chemical compounds based on Fe2Cr or of the eta type carbide (FeW)xC - which

different experimental conditions are presented in **Figure 14**.

some conventional characteristics, obtained after some non-destructive tests.

determination of the hardness by conventional methods, in order to carry out investigations regarding the micro-hardness of the 97MXC deposits, we considered that the most suitable method is the Vickers method. The microhardness represents the Vickers hardness of some elements from the metallographic structure (phases, structural constituents, inclusions, etc.) and of some very thin coatings. The Vickers microhardness values presented in our study were determined using the CV - 400DAT digital microdurimeter, produced by CV Instruments, with a 100 g load, for 10s.

where the absence of the transition zone is observed.

*2.2.4 Microhardness of the 97MXC coatings*

**150**

**Figure 14.**

*HV100 microhardness of 97MXC coatings.*

The tests were performed on three samples of 97MXC coatings obtained by arc spraying process, at pressure p = 6.5 bar and at different values of electric current intensity– see **Table 4**. The turning disk used in tribological tests was made of AISI52100 steel, hardness 64 HRC. The roughness of the tested samples was measured on Taylor-Hobson profilometer. The values of the roughness on longitudinal and transversal direction of tested samples are given in **Table 4**.

**Figures 16** and **17** show the time variations of the friction coefficient and the friction torque at loads of 20 N, respectively 40 N.

The mean values of the coefficient of friction (CoF), as well as the weight loss of the samples after performing the tests at loads of 20 N and 40 N are shown in **Table 5**.


**Table 4.**

*Roughness of the tested samples.*

**Figure 16.** *Friction torque versus time at 20 N.*

**153**

*Hard Alloys with High Content of WC and TiC—Deposited by Arc Spraying Process*

**Sample P1 P2 P3** Load [N] 20 40 20 40 20 40 CoF 0.217 0.207 0.215 0.226 0.242 0.237 Wear, [%wgt] 0.000 0.028 0.000 0.004 0.000 0.011 Initial mass, g 28.976 28.976 47.874 47.855 47.226 47.226 Final mass, g 28.976 28.968 47.874 47.853 47.226 47.221

**Mean friction coefficients CoF**

Analyzing **Figures 16** and **17**, one can be observed that the friction torque in tribological system was more constant for P3 sample, especially at high load - **Figure 17**). Correlated with the wear rate and general friction coefficients (CoF) results from **Table 5**, it can be concluded that the P3 coating assure an almost constant CoF during an hour of continuous testing, while the friction torque of P1

The data presented in **Table 5** show that the CoF values do not vary much by increasing the applied load, respectively from 20 N to 40 N. The average CoF values of the samples were as follows: CoF ≈ 0.21 for P1; CoF ≈ 0.22 for P2; CoF ≈ 0.24 for P3. The CoF varied according to the quality of the tested surfaces of samples, but also with possible increase in temperature over the pad on disk contacts. A higher and constant CoF was obtained for the P3 sample, especially at high load – see

The superior friction behavior of the P3 against P1 and P2 coatings, given by the stability of the friction torque and the low wear rate, can be explained by the presence inside the deposit of the transition area between the W-rich hard phase and the (FeW)xC eta type carbide matrix, which allows fixing and maintaining the hard

The adherence of the deposits obtained by thermal spraying is defined as being the force necessary detaching the layer from the substrate. The studies carried out by Haraga et al. [47] have demonstrated that the adhesion of the coatings is predominantly mechanical and is due to the solidification of the sprayed liquid particles or to the deformation of the semi-viscous particles on the substrate asperities. The adhesion of the deposits was determined by the traction test - in accordance

**Figure 18** presents the adhesion variation of the deposits with the pressure of the compressed air passing through the primary circuit. It is noticed that at low values of the compressed air pressure the layer adhesion to substrate has low values. It can observe that for the same value of the compressed air pressure which passes through the primary circuit adhesion values of the deposit vary with the current intensity. Thus, for the same pressure value, the adhesion of the 97MXC coatings obtained

at I = 250A is superior to the one of the other coatings obtained at I = 200A and I = 220A, in the same technological conditions. Knowing that by the increasing of the current intensity of the electric arc temperature increases, we can suggest that, at high values of the current intensity, the drop formed in the electric arc is atomized into small particles with reduced inertia. Studies carried out by Toma et al. [33], have demonstrated that the high pressure of the compressed air favors the

*DOI: http://dx.doi.org/10.5772/intechopen.94605*

and P2 manifested between large limits.

phase in conditions of advanced wear for a long time.

*2.2.6 Analysis of 97MXC coatings adhesion*

**Figure 17**.

**Table 5.**

*Mean CoF and wear in wt%.*

with EN 582.

**Figure 17.** *Friction torque versus time at 40 N.*

*Hard Alloys with High Content of WC and TiC—Deposited by Arc Spraying Process DOI: http://dx.doi.org/10.5772/intechopen.94605*


**Table 5.**

*Welding - Modern Topics*

The tests were performed on three samples of 97MXC coatings obtained by arc spraying process, at pressure p = 6.5 bar and at different values of electric current intensity– see **Table 4**. The turning disk used in tribological tests was made of AISI52100 steel, hardness 64 HRC. The roughness of the tested samples was measured on Taylor-Hobson profilometer. The values of the roughness on longitudinal

**Figures 16** and **17** show the time variations of the friction coefficient and the

The mean values of the coefficient of friction (CoF), as well as the weight loss of the samples after performing the tests at loads of 20 N and 40 N are shown in **Table 5**.

> **P2 I = 220A**

**P3 I = 250A**

**Disk AISI 52100**

**I = 200A**

Longitudinal roughness, Ra [μm] 14.91 14.35 14.26 0.83 Transversal roughness, Ra [μm] 14.42 14.66 14.78 1.28

and transversal direction of tested samples are given in **Table 4**.

friction torque at loads of 20 N, respectively 40 N.

**Sample P1**

**152**

**Figure 17.**

*Friction torque versus time at 40 N.*

**Figure 16.**

**Table 4.**

*Roughness of the tested samples.*

*Friction torque versus time at 20 N.*

*Mean CoF and wear in wt%.*

Analyzing **Figures 16** and **17**, one can be observed that the friction torque in tribological system was more constant for P3 sample, especially at high load - **Figure 17**). Correlated with the wear rate and general friction coefficients (CoF) results from **Table 5**, it can be concluded that the P3 coating assure an almost constant CoF during an hour of continuous testing, while the friction torque of P1 and P2 manifested between large limits.

The data presented in **Table 5** show that the CoF values do not vary much by increasing the applied load, respectively from 20 N to 40 N. The average CoF values of the samples were as follows: CoF ≈ 0.21 for P1; CoF ≈ 0.22 for P2; CoF ≈ 0.24 for P3. The CoF varied according to the quality of the tested surfaces of samples, but also with possible increase in temperature over the pad on disk contacts. A higher and constant CoF was obtained for the P3 sample, especially at high load – see **Figure 17**.

The superior friction behavior of the P3 against P1 and P2 coatings, given by the stability of the friction torque and the low wear rate, can be explained by the presence inside the deposit of the transition area between the W-rich hard phase and the (FeW)xC eta type carbide matrix, which allows fixing and maintaining the hard phase in conditions of advanced wear for a long time.
